getDbInstance()'s probe-then-reopen pattern (written for per-open-handle
drivers like better-sqlite3/node:sqlite) was calling .close() on a
throwaway probe connection before opening the "real" connection right
after. For sql.js, openSqliteDatabase()'s fallback path always returns
the SAME module-global cached singleton for a given filePath, so closing
"the probe" closed the ONLY connection that file would ever get until
process restart — every subsequent query threw sql.js's raw "Database
closed" string, matching the reported crash-loop on every boot once
storage.sqlite already exists and both sync drivers are unavailable.
Adds closeProbeIfSafe() (src/lib/db/core.ts) and uses it at every
probe-close site in getDbInstance()/captureCriticalDbState() — it skips
the close for sql.js-backed adapters and lets the same live adapter flow
through, while still closing real per-handle drivers normally.
Also makes sqljsAdapter.ts's gracefulClose() remove its 3 process-level
listeners (beforeExit/SIGINT/SIGTERM) so a closed adapter's closure (raw
sql.js Database + buffers) can actually be garbage collected instead of
being pinned forever — addresses the compounding-OOM sub-finding as a
consequence of the same defect.
Regression test: tests/unit/db-sqljs-close-poison-7494.test.ts
