refactor: migrate build and runtime assets to project-local dist

[aofei]

Restructure build and release workflows so SPX artifacts are produced and consumed from dist/ instead of $GOPATH/bin. This makes local builds and packaged goenv releases use the same layout.

Build and packaging:

• add scripts/build.sh, scripts/setup-engines.sh, scripts/setup-web-template.sh, and scripts/verify-goenv.sh
• update Makefile to use build, build-web, build-native, setup, setup-engines, and setup-web
• make run-web and run-web-worker fail fast when build-web fails

Runtime and resource loading:

• resolve ShareDir and LibDir from executable-relative paths
• load engines from dist/share/engines and native libs from dist/lib
• remove spxrun runtime auto-download and require pre-provisioned engines
• track runtime versions with pck_version in cmd/gox/template and cmd/spxrun/runner, and pck_release_tag in cmd/gox/template

Web export cleanup:

• remove brotli-specific .br handling and -build=fast leftovers
• remove isWasmCompressed leftovers and keep uncompressed wasm flow
• improve error propagation in web export and worker merge paths

CI and release:

• switch cache namespace from spx-godot-* to spx-assets-*
• make deps cache and setup arch-aware via spx-arch and SPX_ARCH
• package goenv with go/bin, go/share, and go/lib from dist
• re-enable release runtime verification with explicit success marker checks

Documentation:

• update README.md and docs/zh/dev/engine/cmd_make.md for the new dist-based workflow
• refresh web-related docs under docs/zh/dev/engine/

This is a breaking change. Existing installations that depend on $GOPATH/bin must migrate to the dist/ layout.