feat: batch settlement go sdk

[CarsonRoscoe]

Follow up of PR #2164, which was automatically closed when the Typescript SDK merged to main

Description

Adds the batch-settlement EVM scheme to the Go SDK at parity with the TypeScript reference implementation, plus the
resource-server architectural changes from #2109 that batch-settlement depends on.

New: batch-settlement scheme (go/mechanisms/evm/batch-settlement/)

• Client: BatchSettlementEvmScheme with deposit (ERC-3009 + Permit2 transports), voucher signing, channel-state recovery, corrective
  402 resync, cooperative refund (scheme.Refund(url)).
• Facilitator: simulate-then-execute for deposit/claim/refund/settle; multicall(claim+refund) on cooperative refund; ECDSA + ERC-1271
  voucher verification.
• Server: atomic per-channel UpdateChannel storage + PendingRequest reservations; voucher local-commit short-circuit (no on-chain
  settle for vouchers); local voucher verification when on-chain state is fresh (OnchainStateTtlMs); InMemory + File channel storage
  backends; BatchSettlementChannelManager claim/settle/refund tick loop.
• Wire format matches TS byte-for-byte (nested extra.channelState, chargedAmount, etc.); error codes consolidated under the
  invalid_batch_settlement_evm_* prefix.

Resource-server hook architecture (go/server.go, go/server_hooks.go, go/server_hook_policy.go) — port of TS PR #2109:

• Per-(network, scheme) hook isolation: scheme-provided hooks no longer leak into the global hook list and only fire for the matched
  scheme.
• Hook execution order: manual → matched scheme → declared extensions (mirrors TS getLabeledHooks).
• Extension hook adapters: extensions can declare ResourceServerExtensionHooks{...}; each hook is gated on
  ctx.DeclaredExtensions[key] being set on the route.
• Scheme-level EnrichSettlementPayload / EnrichSettlementResponse providers, called around facilitator settle.
• Additive-policy guards (AssertAcceptsAllowlistedAfterExtensionEnrich, AssertAdditivePayloadEnrichment,
  AssertAdditiveSettlementExtra, MergeAdditiveSettlementExtra, etc.) — full port of TS hookPolicy.ts. Replaces TS's compile-time
  DeepReadonly enforcement with runtime additive-policy enforcement.
• New VerifyPaymentWithExtensions / SettlePaymentWithExtensions / ProcessSettlementWithExtensions /
  CreatePaymentCancellationDispatcherWithExtensions methods thread declared extensions through verify → settle → cancellation.
  Existing API methods delegate with nil for backward compat.
• HTTP middlewares (nethttp, gin, echo) all forward result.DeclaredExtensions and honor the SkipHandler directive
  (previously only nethttp did).

Examples (examples/go/{clients,servers,facilitator}/batch-settlement/): drop-in client, server, and facilitator examples with .env-example files mirroring the TS examples.

E2E (e2e/clients/go-http, e2e/facilitators/go, e2e/servers/{gin,nethttp}): batch-settlement scenarios wired into the cross-SDK harness, exercising deposit + voucher + cooperative-refund phases across go-http × {gin, nethttp} × go-facilitator on Base Sepolia.

Tests

make test and make test-integration from /go both pass:

Cross-SDK e2e testing between Typescript and Go for batch-settlement

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)

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