…ckage RPC + Redis existence key)

- writeSimulationPackage now writes forecast:simulation-package:latest to Redis after
  successful R2 write, containing { runId, pkgKey, schemaVersion, theaterCount, generatedAt }
  with TTL matching TRACE_REDIS_TTL_SECONDS (60 days)
- New getSimulationPackage RPC handler reads Redis key, returns pointer metadata without
  requiring an R2 fetch (zero R2 cost for existence check)
- Wired into ForecastServiceHandler and server/gateway.ts cache tier (medium)
- Proto: GetSimulationPackage RPC + get_simulation_package.proto message definitions
- api/health.js: simulationPackageLatest added to STANDALONE_KEYS + ON_DEMAND_KEYS
- Tests: SIMULATION_PACKAGE_LATEST_KEY constant + writeSimulationPackage null-guard test

Closes todo #17 (Phase 2 prerequisites for MiroFish integration)

… runId mismatch

- Add `error` field to GetSimulationPackageResponse: populated with
  "redis_unavailable" on Redis errors so callers can distinguish a
  healthy not-found (found=false, error="") from a Redis failure
  (found=false, error="redis_unavailable"). Adds console.warn on error.
- Add `note` field: populated when req.runId is supplied but does not
  match the latest package's runId, signalling that per-run filtering
  is not yet active (Phase 3).
- Add proto comment on run_id: "Currently ignored; reserved for Phase 3"
- Add milliseconds annotation to generated_at description.
- Simplify handler: extract NOT_FOUND constant, remove SimulationPackagePointer
  interface, remove || '' / || 0 guards on guaranteed-present fields.
- Regenerate all buf-generated files.

Fixes todos #18 (runId silently ignored) and #19 (error indistinguishable
from not-found). Also resolves todos #22 (simplifications) and #23
(OpenAPI required fields / generatedAt unit annotation).

…no-cache not-found

Three P1 regressions caught in external review:

1. Key prefix bug: getCachedJson() applies preview:<sha>: prefix in non-production
   environments, but writeSimulationPackage writes the raw key via a direct Redis
   command. In preview/dev the RPC always returned found:false even when the package
   existed. Fix: new getRawJson() in redis.ts always uses the unprefixed key AND throws
   on failure instead of swallowing errors.

2. redis_unavailable unreachable: getCachedJson swallows fetch failures and missing-
   credentials by returning null, so the catch block for redis_unavailable was dead
   code. getRawJson() throws on HTTP errors and missing credentials, making the
   error: "redis_unavailable" contract actually reachable.

3. Negative-cache stampede: slow tier caches every 200 GET. A request before any deep
   run wrote a package returned { found:false } which the CDN cached for up to 1h,
   breaking post-run discovery. Fix: markNoCacheResponse() on both not-found and
   error paths so they are served fresh on every request.