feat: add rich source traffic observability

[jakthom]

Summary

• add per-source, sender, and PGN timing, rate, payload, decode, addressing, value-distribution, and raw-wire diagnostics, including unknown PGNs
• surface gaps, bursts, anomalies, identity changes, traffic share, and estimated CAN load in the operator table and beacon_source_pgn_* Prometheus families
• add persistent expected-traffic baselines and bounded lifecycle events, with UI and MCP controls to approve or clear expectations
• expose the shared store through get_source_metrics while keeping raw payload and fingerprint identifiers out of Prometheus labels to bound cardinality

Verification

• go test ./...
• go vet ./...
• go test -race ./internal/stats ./internal/metrics ./internal/mcpserver ./internal/app
• npm test (3 Chromium Playwright tests)
• git diff --check