chore(e2e): make the harness portable to any EVM chain in DEFAULT_STABLECOINS

[ryanRfox]

Closes #2249

Summary

Makes the e2e harness portable to any EVM chain in the SDK's DEFAULT_STABLECOINS catalog. The visible knob is --evm-network=<caip2>; the rest of the PR is the plumbing that makes the knob actually work.

Four interlocking pieces:

1. CLI override — --evm-network=<caip2> selects the EVM chain for a run. Mode-default behavior preserved when the flag is omitted.
2. SDK-imported chain registry — EVM_NETWORK_CONFIGS is derived from DEFAULT_STABLECOINS at module load, replacing the prior 10-entry hand-curated table. New SDK chains propagate after pnpm install with no harness edit.
3. Removes Base Sepolia hardcodes — EIP-712 token names in the four EVM resource servers (express, fastify, hono, next/proxy) move to a per-server lookup keyed by EVM_NETWORK. The Permit2 approval script reads its target from the resolved network config. The mock facilitator advertises the full scheme set (exact, upto, batch-settlement) and gates non-EVM kinds on env presence.
4. EVM-only credential gating — an EVM-only run (--families=evm) no longer requires Solana, Aptos, Hedera, or Stellar wallets. The harness's existing requiredEnvByFamily post-filter check (test.ts line ~660) handles this uniformly across all five families; we removed an unconditional EVM+SVM gate that fired before scenario filtering and over-required both families regardless of --families.

Why

Without (2), the override in (1) has nothing to resolve to. Without (3), selecting a non-Base chain produces wrong EIP-712 signatures, wrong Permit2 setup targets, and incomplete scheme advertisement. Without (4), an EVM-only contributor must set up a Solana wallet they may never use, which raises the entry bar for chain-specific testing.

Concrete consequences this addresses:

1. Chain-specific permit2 personalities — chains whose default asset specifies assetTransferMethod === "permit2" (tokens that ship without EIP-3009) — are unreachable today. Their unique EIP-712 domains, permit signatures, and contract behaviors get no runtime signal. (Note: the harness's permit2 endpoints are exercised on Base Sepolia today because USDC supports both methods; what's missing is the chain-specific personality.)
2. Adding a new SDK chain requires harness source edits. The harness drifts from the SDK's chain catalog over time.
3. Cross-chain regressions are invisible — a SDK change that breaks one chain or assetTransferMethod but not Base Sepolia passes the harness without signal.

After this PR, the harness exercises any chain in DEFAULT_STABLECOINS (including new additions like Radius Network from #2038) through one --evm-network=<caip2> flag, with no harness source edits.

What's in this PR

1. Networks core (e2e/src/networks/networks.ts)

The centerpiece. The hand-curated EVM_NETWORK_CONFIGS table is replaced with Object.fromEntries(Object.keys(DEFAULT_STABLECOINS).map(...)) — a runtime view of the SDK's chain catalog. permit2Asset for any caip2 lookup flows from DEFAULT_STABLECOINS[caip2].address. EVM_NETWORK_CONFIGS is exported so callers can introspect the supported chains.

resolveViemChain(caip2) looks up viem's chain database; for chains viem hasn't packaged it falls through to defineChain({ name: 'EVM <id>', rpcUrls: { default: { http: [] } } }) and the caller supplies EVM_RPC_URL. With the viem floor at ^2.48.11, every chain currently in DEFAULT_STABLECOINS is in viem's catalog, so the fallback is for forward-compatibility with future SDK chains that out-pace viem.

evmRpcUrl(caip2) resolves in three tiers: EVM_RPC_URL env override → viem chain default → ''. getNetworkSet(mode, evmCaip2?) accepts an optional CAIP-2 EVM override that overlays the mode default's EVM slot from EVM_NETWORK_CONFIGS. The mode-default NETWORK_SETS rows for Base / Base Sepolia retain their inline addresses — out-of-scope consolidation candidate.

2. CLI + harness wiring (e2e/src/cli/args.ts, e2e/test.ts)

• --evm-network=<caip2> parsed in parseArgs() as evmNetwork?: string.
• runTest() passes parsedArgs.evmNetwork into getNetworkSet(mode, ...).
• getEvmClients() replaces evmNetwork === 'eip155:8453' ? base : baseSepolia
  with resolveViemChain(evmNetwork).
• Removed an unconditional EVM+SVM env-vars check that fired before scenario
  filtering and over-required both families regardless of --families. The
  existing requiredEnvByFamily table-driven check (same file, line ~660)
  already handles family-conditional credentials uniformly across all five
  protocol families — --families=evm runs no longer require Solana wallets.

3. EVM resource servers

Each EVM-payable server constructs an EIP-712 payment domain. All four sites
previously embedded the binary EVM_NETWORK == "eip155:84532" ? "USDC" : "USD Coin"
ternary at multiple call points. Replaced with a per-server
EVM_PERMIT2_ASSET_NAMES: Record<string, string> lookup keyed by EVM_NETWORK,
with comments indicating where to add entries when new chains land.

• e2e/servers/express/index.ts
• e2e/servers/fastify/index.ts
• e2e/servers/hono/index.ts
• e2e/servers/next/proxy.ts
• e2e/servers/mcp-typescript/index.ts — eip155:* network-prefix widening only

The exported resolveViemChain from networks.ts is consumed by e2e/test.ts.
The clients, the facilitator, and the Permit2 script keep their own local
chain-resolution helpers (each spawned process is its own pnpm sub-package and
imports from e2e/src/... cross workspace boundaries). All local helpers
mirror resolveViemChain's viem-DB lookup with a minimal defineChain fallback.

4. EVM clients

Each client previously selected viem/chains base vs baseSepolia via
evmNetwork === 'eip155:8453' ? base : baseSepolia. Replaced with a local
resolveEvmChain(network) helper that walks viem's chain database and falls
back to a minimal defineChain for chains viem hasn't yet packaged, so any EVM
chain in DEFAULT_STABLECOINS works.

• e2e/clients/fetch/index.ts
• e2e/clients/axios/index.ts
• e2e/clients/mcp-typescript/index.ts — eip155:* network-prefix widening only

5. Facilitator + Permit2 + mock + generic-server

• e2e/facilitators/typescript/index.ts — local getEvmChain(network) helper
  for createPublicClient (mirrors resolveViemChain's lookup pattern). SVM
  signer initialization is now lazy, gated on SVM_PRIVATE_KEY presence so
  EVM-only runs don't require it. Scheme registration is family-conditional.
• e2e/scripts/permit2-approval.ts — reads target token from
  EVM_PERMIT2_ASSET (set by generic-server.ts from the resolved
  network config) with [tokenAddress] CLI override. Works on any chain
  in DEFAULT_STABLECOINS.
• e2e/mock-facilitator/index.ts — advertises batch-settlement alongside
  exact/upto. Non-EVM kinds are gated on env presence so EVM-only runs
  see a coherent /supported response (matches the real TS facilitator).
• e2e/src/servers/generic-server.ts — wires EVM_NETWORK, EVM_RPC_URL,
  and EVM_PERMIT2_ASSET into spawned server processes from the resolved
  NetworkConfig.

6. Package wiring + README

• e2e/package.json — adds "@x402/evm": "workspace:*" (was reachable
  transitively via the workspace, now declared at the e2e top-level package).
• e2e/pnpm-lock.yaml — workspace link side-effect.
• e2e/README.md — documents --evm-network=<caip2>, the EVM_RPC_URL
  single-override env var, the SDK-import auto-propagation, and the
  EVM-only credential gating.

Tests

• pnpm install --frozen-lockfile clean across typescript/, examples/typescript/, and e2e/.
• tsc --noEmit from e2e/ (after pnpm -C typescript build) introduces zero new errors vs upstream/main; the 51 pre-existing top-level-await / missing-route-import / implicit-any errors are unchanged.
• Module-load introspection of EVM_NETWORK_CONFIGS: 12 keys, names sourced from viem's chain database, RPC URLs populated for every chain in DEFAULT_STABLECOINS. New SDK chains propagate after pnpm install with no harness source edit.
• On-chain harness slice on Base Sepolia (--testnet --families=evm --facilitators=typescript --servers=express --clients=fetch --min): 11/11 scenarios pass, exit 0, 28 settlement tx hashes status=0x1 via paced JSON-RPC (1.2 s sleep, sequential, no sampling). Run with no SVM env vars set — confirms requiredEnvByFamily correctly skips SVM credentials on --families=evm.
• On-chain harness slice against a non-Base EVM chain via --evm-network=<caip2> --families=evm: the chain-pluggable path exercises end-to-end with EVM-only credentials. Zero Missing required environment errors reference any SVM variable.
• viem floor is ^2.48.11; e2e/ lockfile resolves direct viem to 2.48.11. The complementary monorepo-wide floor bump is in chore: bump viem floor to ^2.48.11 across monorepo + refresh lockfiles #2242.

Migration

CI configurations that set per-chain RPC URL env vars (e.g.,
BASE_SEPOLIA_RPC_URL) must migrate to the unified EVM_RPC_URL. The harness
selects the chain via --evm-network=<caip2> (or the mode default) and reads
RPC override from EVM_RPC_URL only.

No impact on SDK consumers using the published packages.

Follow-up (not in this PR)

• Codified drift-assertion test for the SDK-import (currently verified manually via module-load introspection).
• Mock facilitator non-EVM kind parity with the real TS facilitator (advertised schemes are now in sync; non-EVM kind coverage may still diverge for AVM/Hedera/Stellar).
• Consolidate the inline Base / Base Sepolia rows in NETWORK_SETS to derive from EVM_NETWORK_CONFIGS for full SDK-import symmetry.

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• I added a changelog fragment for user-facing changes (docs-only changes can skip) — internal refactor, no @x402/* runtime API surface changes

AI disclosure

This PR used an agentic coding workflow and was reviewed by Ryan R. Fox (an actual human) before posting.

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