feat(p2p): DHT hardening, cover traffic module, and config hot-reload

[aratan]

Summary

• Seed health gate with degraded mode (GossipSub-only) when DHT unreachable
• Parallel mDNS + DHT bootstrap for faster LAN peer discovery
• DHT health monitor with 2-failure threshold and auto-recovery
• Dedicated cover traffic module on /28sp/cover/v1 with CPU throttle
• InitCoverTraffic/StartCoverTraffic public API on P2PHost
• ConfigWatcher with SHA-256 polling (2s) for hot-reload of cover rates
• 8 RED tests (covertraffic) passing, 5 P2P test stubs

Changes

File	Change
internal/covertraffic/covertraffic.go	NEW — Cover traffic module with Start/Stop/SetRate/CPU throttle
internal/covertraffic/covertraffic_test.go	NEW — 8 unit tests for cover traffic (all passing)
internal/p2p/hotreload.go	NEW — ConfigWatcher with SHA-256 polling
internal/p2p/p2p_test.go	NEW — 5 RED test stubs (seed health, DHT degradation, cover, watcher)
internal/p2p/p2p.go	Seed health (Config), DHT health monitor, degraded mode, cover traffic wiring
internal/p2p/discovery.go	checkSeedHealth() with parallel 5s timeout
main.go	InitCoverTraffic + StartCoverTraffic calls
config.yaml	seedAddrs, requireSeeds fields added

Test Plan

• go test ./internal/covertraffic/... — 8/8 PASS
• go build ./... — compiles clean
• go vet ./... — no warnings
• go build ./internal/p2p/... — builds (tests require libp2p, longer compilation)

SDD

This is PR 1/4 of SDD change integrar-28sp-en-mesh-hermes.