fix(runtime): claim blocked select cases atomically

[paralin]

Blocked select branches could consume multiple channel values before Promise.race chose a winner. Claim selection synchronously at channel transfer, cancel losing cases before further transfers, and prevent a select from rendezvousing with itself.

Validation: 33 runtime contract tests, compiled select compliance fixture, TypeScript typecheck, and JavaScript lint pass. The formerly intermittent downstream WebKit recovery test passes 50 repetitions in Chromium and WebKit. Go lint could not read the installed Go 1.27 export format; its binary supports an older format.

[Copilot]

🔵 Needs a closer look

It changes core channel/select concurrency semantics and scheduling behavior in the runtime, which is high-impact and warrants final human verification beyond automated review.

Pull request overview

This PR fixes a race in the GoScript runtime select implementation where blocked branches could consume multiple channel values before Promise.race resolved, by claiming selection synchronously at the channel transfer point, aborting/withdrawing losing cases immediately, and preventing a select from rendezvousing with itself.

Changes:

• Extend selectReceive/selectSend with an onCommit hook and use an AbortController to cancel/withdraw losing cases atomically at commit time.
• Adjust channel rendezvous/wakeup scheduling to avoid extra microtask deferrals that allowed other transfers to slip in before commit.
• Add regression coverage in both runtime contract tests and the select compliance fixture (plus expected output), and document the updated selectStatement behavior.

File summaries

File	Description
gs/builtin/channel.ts	Implements atomic commit for blocked select cases via onCommit + abort/withdrawal, and enforces “no self-rendezvous” within a single select.
gs/builtin/runtime-contract.test.ts	Adds targeted runtime contract tests covering losing-branch preservation, send withdrawal, and self-rendezvous prevention.
tests/tests/select_statement/select_statement.go	Extends the Go compliance fixture with concurrent send/select scenarios to detect value loss/double-consumption.
tests/tests/select_statement/select_statement.gs.ts	Updates the compiled TypeScript fixture to match the new Go test and validate runtime behavior end-to-end.
tests/tests/select_statement/expected.log	Adds the new expected compliance output line for the added test case.
design/DESIGN.md	Documents the updated blocked-select claiming/cancellation behavior and self-rendezvous rule.

Review details

• Files reviewed: 5/6 changed files
• Comments generated: 0
• Review effort level: Lite

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