Expose Wasmtime cache write degradation

[lthibault]

The persistent Wasmtime cache reports enabled after startup validation, but later filesystem write failures (for example, a full or read-only PVC) can silently degrade future boots to uncached compilation.

Add an operator-visible signal for cache write failures and decide whether it should affect ww healthcheck, readiness, or only the existing cause monitor. Keep the POC posture in mind: local logs and metrics are sufficient; no external alert endpoint is required.

Acceptance criteria:

• failed cache writes increment a documented metric or status field;
• the failure path has focused coverage;
• monitor/health behavior is explicitly documented.