[runtime] Short-circuit cached-only /verify schema checks early

[GsCommand]

Motivation

• Cold schema compilation was blocking POST /verify?schema=1 long enough to cause client aborts instead of returning the documented fast 202 warmup response.

Description

• Move extraction of verb and add an early cached-only guard in POST /verify (in server.mjs) so that when wantSchema && VERIFY_SCHEMA_CACHED_ONLY && !hasValidatorCached(verb) the endpoint immediately enqueues warmup work and returns 202 with validator_not_warmed_yet before any schema fetch or AJV compilation happens.
• Remove the later cached-only branch from the in-flow schema validation so cold compilation paths cannot start before the short-circuit decision.
• Preserve existing proof/hash/signature verification behavior and only short-circuit schema-related work; add reason: "validator_not_warmed_yet" to the warmup response for the chain test retry path.

Testing

• Ran npm run check and the syntax check passed.
• Ran targeted unit tests with node --test runtime/tests/runtime-signing.test.mjs and all tests passed.
• Ran full test suite with npm test (unit + smoke) and all tests passed.

---

Codex Task