RFE: Backend bounded application caching with expiry and metrics #5996

The backend has security-specific stores and caches, but no general bounded application-cache abstraction comparable to the common JVM cache libraries. Applications need predictable expiry, concurrent loading, invalidation, and memory limits without assuming those libraries translate to the native runtime.

Requested capability

• Provide typed caches with maximum size or weight, expire-after-write/access, and optional per-entry expiry.
• Define atomic lookup/load behavior, concurrent load deduplication, invalidation, and removal notifications.
• Integrate hit/miss, load, eviction, and size metrics with the existing backend metrics API.
• Document lifecycle, loader failure, listener execution, and process-local scope.

Acceptance criteria

• Concurrent callers do not trigger unintended duplicate loads for the same key.
• Expiry, size limits, replacement, invalidation, and removal notifications have deterministic tests using a controllable clock.
• Loader/listener exceptions cannot corrupt the cache or leak request state.
• The same cache API works on the JVM and packaged native backend, with a documented memory overhead.

Current implementation

• vm/backend/src/com/codename1/backend
• docs/developer-guide/Backend-Observability.asciidoc

Validate behavior on the JVM and locally on the packaged ParparVM backend where applicable. This request does not call for native-image or GraalVM CI jobs.