test(memoryOps): stop the memory-op tests writing into the real machine-wide index

[davidj4tech]

The leak

applyMemoryOps() mirrors every write into the machine-wide PGlite memory index
via the fire-and-forget indexMemoryWrite() → upsertMemoryEmbedding(), keyed by
repoKey(stateDir). For this file's tmp state dirs that key is the tmp path
itself, so running src/memoryOps.test.ts writes rows into the developer's real
~/.pi/mega-compact-vector:

/tmp/mc-memops-w3XL9J/add      "we use node:sqlite as the store"
/tmp/mc-memops-w3XL9J/dup      "threshold is 50k"
/tmp/mc-memops-w3XL9J/replace  "the threshold is 100k"

They persist, and they are reachable. Cross-repo recall is on by default
(crossRepoEnabled: true), recallMemoriesCrossRepo() searches the global index
with no repo filter, and the floor it uses is opts.crossRepoCosine ?? 0.3 —
no caller sets it, so the strict crossRepoCosine: 0.90 from config never reaches
this path (it is passed as dedupSim in extensions/mega-pipeline/recall.ts).

So a live session in an unrelated project can be handed a stale test assertion as
if it were another project's memory — including the contradictory 50k/100k pair.
This was found on a real machine whose index held 33 such rows and nothing
else: every entry available to cross-repo recall was test junk.

scripts/run-tests.mjs already sets MEGACOMPACT_INDEX_DIR per child, so the
suite was never the leak — invoking the file directly was, which is exactly what
one does while debugging.

The change

Disables PGlite for the file — the same guard memoryRoundtrip.test.ts already
uses for the same reason, since these suites exercise the sync node:sqlite path
only — and points MEGACOMPACT_INDEX_DIR at the per-run tmp dir so the isolation
still holds if a later test re-enables the index.

Verification

	live-index rows added	result
current code, one run	3	9/9 pass in 11s
this change, three consecutive runs	0	9/9 pass in <1s each

The file also stops emitting "asynchronous activity after the test ended" — with
a per-run index dir, an initdb was still running when the first test finished.

Purely test-side; no src/ behavior change.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.