Guard every async socket handler against an escaping rejection

[atomantic]

Summary

• New guard: server/sockets/asyncHandlerGuard.test.js. Every async Socket.IO handler in server/sockets/ must own its rejections — a handler runs outside the Express request lifecycle and nobody holds the promise it returns, so a rejected await becomes an unhandled rejection and kills the server process (that is how voice:call:detach could have gone down, voice:call:detach is the one async socket handler in voice.js with no try/catch, so a failed detach takes the server down #5661). The guard scans the directory, and a failure names the file, the line and the event string.
• Scope is every emitter in the directory, not just socket.on — ns.on('connection', …) and the pm2 logs child's .on('data', …) in logs.js sit in the identical blast radius.
• One shared implementation instead of a second copy. server/timerCallbackConventions.test.js already had a hardened lexer, bracket matcher, callback parser and await checker for the same rule on timer callbacks. Rather than duplicating it, that machinery moved to server/lib/sourceScan.js (barrel + README row) and both guards now consume it, so the timer rule and the socket rule cannot drift on what "owns its rejection" means. childProcess.guards.test.js takes its blankComments from there too.
• Three bypass probes, because a source scan that silently matches nothing passes green forever: a bare await is flagged; a try that exists only in a comment does not satisfy the guard; and each handler body is captured by a bracket-balanced walk, so an unguarded handler cannot pass on its neighbour's try (a fixed character window reads the next statement).
• One review finding fixed in the same diff: the callback parser skipped a ( found after an arrow's =>, reading it as a parameter list, so async () => (await f()) had its whole body jumped and every await in it went unseen. That predates the extraction — the timer guard carried the same line — and it exempted a real spelling from both guards. Both recognizers now cover the shape.
• No source fix needed. The one offender the issue named (voice:call:detach) was fixed in 106d53d5d (voice:call:detach is the one async socket handler in voice.js with no try/catch, so a failed detach takes the server down #5661) before this landed; every one of the 19 async handlers is green today. Documented in server/AGENTS.md beside the timer rule, and routed in scripts/ci-test-plan.js so an edit under server/sockets/ selects the guard (it readdir-scans rather than imports, so no import edge reaches it).

Test plan

• cd server && npm test — full suite green.
• npx vitest run sockets/asyncHandlerGuard.test.js timerCallbackConventions.test.js lib/index.test.js lib/childProcess.guards.test.js ../scripts/ci-test-plan.test.js ../scripts/repo-scan-guards.test.js
• Mutation-verified: stripping the try/catch from voice:call:detach makes the guard fail with server/sockets/voice.js line 543 'voice:call:detach': await detachHost(socket); restoring it goes green.
• The timer guard's 13 existing tests pass unchanged against the extracted module, which is the regression check on the extraction.

Closes #5678