Refuse a non-integer snapshotId as malformed, not stale

[Ayush7614]

What this changes

asRef and POST /:botId/control/secret accepted any typeof === number as a snapshot id. 1.5 and Infinity (valid JSON: 1e999 parses to it) then never equalled the stored integer, so the gateway reported a stale snapshot (409) plus an allowed-then-failed audit pair, and the caller retried a request that was malformed. Both sites now require Number.isInteger; malformed ids answer the existing 400 bad-ref before any decision or audit row.

Covers the acting routes through asRef (click/type and the rest) and the secret route which checks inline.

Where it runs

• New state that outlives a request? None. Request-shape check at the route edge.
• What happens on the second replica? Same 400 on every replica: no stored state consulted, no coordination needed. Stale-snapshot comparison afterwards still reads the snapshot store, unchanged.
• Anything serialised? No. Refusal happens before gateway resolve/decide/audit.
• Anything fanned out to a browser? No.
• New listener, port, or schedule? No.

Boundary and audit

• Every acting call still goes through the gateway: resolve, decide, audit, then act. Malformed input is refused before the gateway, as before; well-formed input flows unchanged.
• New refusals and new failures each write a row. N/A by design: a 400 for malformed input writes no audit row, matching the existing bad-ref path (only decided actions are audited).
• Nothing new is trusted from the client that the server can resolve itself. The snapshot id is still compared against the server-held snapshot afterwards.

Changelog

• A line in CHANGELOG.md under Unreleased: fractional/infinite snapshot id entry.

Proof

• bun test server/tests/computer-snapshot-id.test.ts: 7 pass (integer reaches gateway; float, Infinity, -Infinity, string, null, boolean each 400 with the bad-ref message and zero gateway calls).
• bun test server/tests/computer-scroll-delta.test.ts: still green (neighbouring edge-validation suite).
• bun run format:check: clean. bun run lint: clean.
• Server typecheck passes (bun run typecheck server/workspace OK; app has the pre-existing embla-carousel-react failure identical on clean main).

[davidmckayv]

Deep-reviewed clean (validation, no secret leak, fail-closed, agrees with existing layers). CI green.