doublezerod: add periodic kernel route reconciliation

[nikw9944]

Resolves: #3669

Summary of Changes

• Add a periodic route reconciliation goroutine to the liveness manager that scans the kernel routing table every 30s (configurable via --route-liveness-reconcile-interval), detects BGP routes that should already be installed but are missing, and reinstalls them
• This mitigates the case where another process or an administrator mistakenly deletes a doublezero route from the kernel routing table
• Add doublezero_liveness_route_reinstalls_total and doublezero_liveness_route_install_failures_total Prometheus metrics to track reinstalls and failures
• Prevent TOCTOU race in active mode: re-check installed state under lock before each reinstall so reconcileRoutes cannot resurrect a route that onSessionDown intentionally withdrew between snapshot and reinstall
• Include source IP in the kernel route lookup key so routes with the same (table, dst, nexthop) but different source IPs are matched independently in multi-interface setups
• Promote "session down (passive; keeping route)" log messages from Debug to Info, otherwise it's possible to log multiple 'liveness: session up' messages in a row

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	1	+116 / -2	+114
Scaffolding	2	+30 / -13	+17
Tests	1	+124 / -0	+124
Docs	1	+2 / -0	+2
Total	5	+272 / -15	+257

Bulk of the change is the reconciliation loop and its tests.

Key files (click to expand)

• client/doublezerod/internal/liveness/manager.go — reconcileRoutes() implementation with TOCTOU guard and src-aware kernel key, config field + validation, goroutine launch, startup log, Debug→Info log level change
• client/doublezerod/internal/liveness/manager_test.go — unit tests for route reconciliation (missing route reinstall, present route skip, uninstalled route skip, install failure metric)
• client/doublezerod/internal/liveness/metrics.go — RouteReinstalls counter and routeReinstall helper
• client/doublezerod/cmd/doublezerod/main.go — --route-liveness-reconcile-interval flag wiring with named default const

Testing Verification

• Unit tests cover: reinstalling a missing route, skipping a route present in kernel, skipping an uninstalled route (active mode, session never went Up), and incrementing the install failure metric on RouteAdd error
• Tests use mock Netlinker to simulate kernel route state; reconciliation ticker set to time.Hour in tests to prevent background interference while calling reconcileRoutes() directly
• Full liveness test suite passes (40s, all existing tests unaffected)
• go vet and go build clean