BLA-4010 add build cache operation observers

[shreyas-blacksmith]

Summary

Adds generic build-cache operation observation hooks to the Blacksmith bazel-remote fork. This is intentionally FA/Web agnostic: bazel-remote only forwards opaque labels and semantic cache outcomes.

Metric contract

• action_cache_get: hit | miss | error
• cas_lookup: hit | miss | error
• backend_upload: error | dropped

Client transfer bytes are not inferred in bazel-remote. FA observes gRPC payloads and emits client_upload / client_download rows.

Safety

• Nil observer preserves current behavior.
• Observer panics are swallowed, so metrics cannot affect cache serving.
• No FA, Web, Laravel, tenant, repo, or job concepts are interpreted in this repo.

Tests

• env GOCACHE=/private/tmp/codex-go-cache-bazel-remote go test ./cache/...
• git diff --check

---

^{Need help on this PR? Tag /codesmith with what you need. Autofix is disabled.}

---

^{Need help on this PR? Tag /codesmith with what you need. Autofix is enabled. (Staging)}

[linear-code]

BLA-4010