perf(hotpath): add cpu_hotspot_ring bench and send-chain optimization plan

- Add measure_all to PeerMap and CidrSet impl blocks (hotpath::measure_all)
- Add [profile.hotpath] for samply-compatible builds (strip=false, debug=line-tables-only)
- Add cpu_hotspot_ring example: 2-node ring tunnel with data-plane flooding (~234K pps)
- Add plans/006-send-chain-cpu-optimization.md based on hotpath+samply 423M sample analysis
  Key findings: dashmap redundancy (14.9%), metrics overhead (8.3%), mpsc (14.1%)
  Target: reduce send_msg_internal from 3.26us to ~2us per packet

Cargo.toml

@@ -27,3 +27,9 @@ lto = true
 codegen-units = 1
 opt-level = 3
 strip = true
+
+# For hotpath CPU profiling: samply needs debug symbols and unstripped binaries.
+[profile.hotpath]
+inherits = "release"
+strip = false
+debug = "line-tables-only"