fix(runtime): unref the game-state watcher so it can't pin the event loop

[davidj4tech]

The bug

ensureGameStateWatcherImpl() opens one fs.watch per MegaRuntime. An
fs_event handle that is active and referenced holds node's event loop open,
and only dispose() released it — so any process that never reaches
session_shutdown finishes all its work and then sits there forever.

That is exactly what the extension test files do. A node diagnostic report
(node --report-on-signal --report-signal=SIGUSR2) taken at hang time shows ten
referenced fs_event handles, one per runtime the file constructs:

fs_event  is_active=True  is_referenced=True     x10
async     is_active=True  is_referenced=False
timer     is_active=True  is_referenced=False

The watcher is a cache-eviction convenience, never a reason to keep the process
alive — so it is now unref()'d at creation, exactly like the perf sampling
interval. While pi is running there are always other referenced handles, so the
watcher still fires normally; dispose() still closes it for prompt fd release.

Result

Run directly, each file exiting on its own (the file exiting is the assertion):

file	before	after
extensions/mega-runtime/state.test.js	never exits	4s
extensions/mega-cache-replay.test.js	never exits	6s
extensions/mega-teamrun.test.js	never exits	13s

Why this hid for so long

process.getActiveResourcesInfo() does not report fs_event handles, so at
hang time it shows only stdio pipes — which reads as "a pending promise, no open
handle" and sent earlier investigation toward PGlite.
MEGACOMPACT_PGLITE_DISABLED=true did make the files exit, but only because that
path never reaches bindRepo() → ensureGameStateWatcher().

Relationship to #5 and #6

This is a third, independent holder of the event loop, and it does not
supersede either earlier fix. I re-measured #5's premise to be sure: opening both
indexes and then closing them exits in ~3s, while leaving them open either stalls
~13s or never exits at all — so closing on shutdown remains necessary. #6's
bounded open is untouched by any of this.

The second commit is comment-only. The note added alongside #5 stated that
dispose() "only released the fs.watch handle and the perf interval, neither of
which was the culprit". The fs.watch handle was a culprit, so that comment is
corrected in place rather than left to mislead the next reader; the measured
close-vs-leave-open numbers replace the hand-wavy phrasing. The perf interval
genuinely is not a culprit — it is unref'd.

Not fixed here

extensions/mega-compact.test.ts still hangs, but on an unrelated defect: it
wedges inside its final cleanup test at ~99% CPU, blocked in PGlite's
synchronous execProtocolRawSync on the INSERT INTO memory_index from
upsertMemoryEmbedding(). Because the loop is blocked, the
Promise.race(…, setTimeout(3000)) guard in that test can never fire. Ruled out
so far: zero/NaN vectors, duplicate vectors, bulk inserts, close racing an
in-flight upsert, use-after-close, and the data itself (replaying the file's real
tuples into a fresh index runs at 1-2ms each). Worth its own issue.

Also unrelated and pre-existing: the test "S28: non-length stopReasons do not arm
the length-stop flag" fails identically on unmodified master.

[gemini-code-assist]

Caution

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