(index): pinned-read null loop and one SQLite test shim #868

Follow-ups to #740 / #859 (round-c review approved).

1. Null full read loops on the budget (session-cache.js ~553-554 with ~629-640). The shared continuation budget is zeroed before readSessionFile; if the read returns null (read error, or a same-mtime rewrite left with no summary/messages), nothing is upserted, the cached fileSize stays old, and every later walk rereads the file (up to 1 MiB) and spends the whole budget again while the row stays stale. Fix: when the pinned full read returns null, adopt the size (sizesToAdopt) or fall through to the header path. Test with readSessionFile returning null for a pinned, grown row.
2. Duplicate SQLite test shims: test/fixtures/harness-store.cjs (from (harnesses): CLI harness port and Claude adapter, no behaviour change (#820) #865) repeats the node:sqlite shim of test/fixtures/conversation-store.cjs; test/session-meta-rekey-696.test.js inlines it too. Give conversation-store.cjs a loader accepting { readonly } and reuse it in both.