feat(api): cursor fingerprinting — CURSOR_STALE (policy R5)

[Levalicious]

The last piece of docs/api-error-policy.md: pagination can no longer serve a silently shifted page.

What changed

• Cursors are now "<index>:<fnv32(result set)>" — the fingerprint is computed over the serialized bytes pagination boundaries are cut on. If the set changed between pages (content or order), the request fails with a visible CURSOR_STALE telling the caller to re-run and page from the start. Malformed cursors are rejected the same way; bare-number cursors are still accepted for legacy callers (unchecked, documented).
• Prerequisite fix: the sort functions tie-broke with Math.random(), which made any multi-page drain of a default-sorted list inherently nondeterministic — the exact silent-corruption class R5 exists to catch. Tie-breaks are now deterministic (by name), so result sets have a stable total order. This also explains several historical pagination false alarms in the KB.
• nextCursor type: string | null; cursor schemas accept number | string.

Evidence

• New API cursor integrity (R5) tests: stale-after-mutation → CURSOR_STALE; unchanged set drains cleanly via string cursors (no dupes, no gaps); malformed cursor rejected; legacy numeric cursor still works.
• Full suite 196/196; fuzz clean (protoErr = 0).

KB notes at release time: H_PaginationFalseAlarm loses its "exhaust cursors" caveat once sets are fingerprinted, and absence conclusions become safe for paginated reads.

🤖 Generated with Claude Code

[github-actions]

Per-op benchmark — p50 cycles (base vs head)

base runs: 5, head runs: 5 (median of mins) · freq-correction ×1.0062 · threshold head−base > max(40 cyc, 5%)

op	base	head*	Δ	Δ%
lookup	115.9	114	-1.9	-1.6%
read_entity	40	40	+0.0	+0.0%
neighbors_d1	850.5	831	-19.5	-2.3%
neighbors_d2	2658.2	2632	-26.2	-1.0%
find_path_d6	175298	188567	+13269.0	+7.6%	🔴 REGRESSION
search	1186756	1186943	+187.0	+0.0%
entities_by_type	14038	14126	+88.0	+0.6%
random_walk_d5	1479.5	1504	+24.5	+1.7%
inc_walker_visit	17.5	18	+0.5	+2.9%
structural_sample	2574019	2594208	+20189.0	+0.8%
compute_merw_psi	912233	925380	+13147.0	+1.4%
create_entity	857	863	+6.0	+0.7%
create_relation	808	838	+30.0	+3.7%

* head frequency-corrected to base via the reference op.

Regressions detected.