Core code treats one query() page as the complete result set

[cuibonobo]

Summary

query() always paginates — an absent limit returns one adapter-default page, never every match — but several internal call sites in packages/core/src/stack.ts assumed a single call was exhaustive:

• ScopedStack.query()'s grant prefetch and hasGrant()'s non-prefetched path: only the first page of _grant@1 records was ever considered, so a grant past page one was silently invisible — legitimate access denied with no error.
• Stack.deleteAttachment()'s fallback metadata scan: on adapters without contentFieldQuery, only the first page of _attachment@1 records was scanned, so metadata beyond page one survived bytes deletion — an orphan.
• ScopedStack.getAttachment()'s uploader check: hardcoded limit: 1 even on the in-memory-matching path, so it could only ever see one of the requester's uploads. This one didn't need 50+ records to misfire — it false-denies with as few as two uploads.

Changes

• Added queryAllPages() — a cursor-walking helper (throws StackQueryError rather than silently truncating past a generous safety cap) — and applied it at the three sites above. Left migrateAll()'s existing cursor walk as-is: it interleaves per-page mutation with fetching (already-migrated records naturally drop out of the filter as it goes), so "collect everything first" doesn't fit its access pattern.
• Kept the contentFieldQuery fast path everywhere it applies (a narrowed query already terminates in one page) — the full scan only runs on adapters without that capability.
• Brought MemoryAdapter (core's test double) up to real filter parity — content, tags, hasAttachment, relatedTo, appId, date ranges, and payload-aware dissociate() — mirroring sqlite-shared's buildWhereClause, so permission-logic tests exercise real predicates instead of an adapter that quietly ignored most filters.
• Named the "query() always paginates" rule in docs/spec.md §Queries — it was previously unstated, which is how the ambiguity crept in.

Test plan

• Added regression tests: a grant beyond the first page (>50 _grant records) is still honored; deleteAttachment() leaves no orphaned metadata when the match is beyond the first page (>50 records); a new ScopedStack.getAttachment test suite (previously uncovered) including the n=2 uploader-access regression.
• Verified each new test fails against the pre-fix code (temporarily reverted the stack.ts fix and confirmed all three failed with the expected symptoms) and passes with it restored.
• pnpm -r build
• pnpm -r typecheck
• pnpm -r lint
• pnpm -r test (all packages green)
• pnpm format:check

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