docs: update all documentation for V8 rebuild and new API surface

[yneves]

Summary

• docs/maintaining.md: Added full "V8 rebuild via GitHub Actions" section (matrix, GN args, why gcc on linux/amd64, archive splitting, assemble job). Expanded local rebuild section. Added "Platform-specific linker notes" (--start-group, gcc vs clang, use_thin_lto). Updated fork files conflict table (+10 entries).
• docs/api-reference.md: Updated ArrayBuffer/SandboxEnabled descriptions for sandbox-disabled deps. Added missing helpers: StrictEquals, TypeOf, MarshalJSON, NewInstance, RetainedValueCount, IsExecutionTerminating.
• docs/architecture.md: Added V8 build pipeline section with mermaid diagram and platform decision tree. Updated CGO file map. Added sandbox-disabled implications.
• docs/release.md: Removed stale auto-bump-downstreams references. Added consolidated workflow inventory table.
• docs/performance.md: Updated zero-copy ArrayBuffer section (sandbox now disabled, no caveat).
• README.md: Updated fork additions list, added V8 rebuild section with CLI examples.
• CHANGELOG.md: Expanded chess.5 with V8 sandbox disabled, fork-maintained deps, and build infrastructure entries.
• docs/zero-cold-start.md: Noted sandbox is now disabled.

Test plan

• CI passes (docs-only change, no code modifications)