runtime: surface transient /verify timeouts as retryable 502 availability failures

[GsCommand]

Motivation

• /verify requests that time out were being reported as generic application failures that looked like cryptographic proof failures, causing callers to treat valid receipts as invalid.
• The runtime must expose availability vs cryptographic failures so frontends can show retryable messages and not mark receipts invalid.
• This repo contains the runtime /verify surface (no browser frontend), so the minimal change is to the API response and logging so clients can distinguish transient failures.

Description

• Change /verify timeout handling in server.mjs so when verify times out (error verify_timeout) the server returns HTTP 502 with failure_type: "availability", retryable: true, reason: "verify_service_unavailable", and a user-facing message explaining the retry recommendation, and sets proof checks fields to null.
• Add a transient availability log entry with context (want_ens, want_schema, refresh, strict_kid, verb, signer_id) to help operators diagnose ENS/schema/ proxy or process stalls.
• Preserve existing semantics for cryptographic failures and schema errors: signature/hash/schema failures still produce the canonical verification shape and status codes.
• Add a unit/integration test in runtime/tests/runtime-signing.test.mjs that simulates a stalled ENS/RPC path and asserts the 502 availability response and new fields.
• Document the new behavior in README.md and docs/OPERATIONS.md so clients/operators treat 502 as transient availability (not proof invalidation) and know likely runtime causes to inspect.

Testing

• Ran the repo test suite with npm test and it completed successfully (all tests passed, including smoke checks).
• Ran the targeted runtime tests with node --test runtime/tests/runtime-signing.test.mjs and the new availability test passed.
• The new test verifies that a stalled ENS/RPC path with ens=1&refresh=1 plus a short VERIFY_MAX_MS results in a 502 response, ok: false, failure_type: "availability", retryable: true, and nullified proof checks.

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Codex Task