feat: add protected FDB summary collector

[rokernel]

Summary

• add a new FDB collector that reads ASIC DB entries and exports summary metrics for total entries, entries by port, entries by VLAN, and entries by type
• keep the scrape path protected by using background refresh with cache, timeout, max-entry limits, and truncation/skipped counters
• add ASIC DB support in the Redis client and register the new collector behind FDB_ENABLED (default disabled)
• add fixtures and tests for FDB parsing, unknown VLAN handling, per-port/per-vlan counts, and truncation/skipped tracking
• document new FDB collector and configuration variables in README

Why

Operators need MAC/FDB visibility for Grafana dashboards and alerting, but exporting per-MAC labels would create high cardinality. This change provides useful aggregate metrics while protecting switch and Prometheus performance.

Verification

• go test ./...
• go build ./...
• go run github.com/golangci/golangci-lint/cmd/golangci-lint@v1.64.8 run
• ./scripts/build.sh