twamp: add timestamp diagnostic tool (#3380)

## Summary of Changes
- Add `twamp-debug` CLI tool for diagnosing kernel timestamping support
on Arista switches
- Tests each SO_TIMESTAMPING mode (RX/TX software/hardware/sched) by
sending real TWAMP probes and checking whether timestamps are actually
delivered in cmsgs, rather than just checking setsockopt acceptance
- Sends configurable probe runs comparing userspace, SO_TIMESTAMPNS,
SO_TIMESTAMPING software, and hardware timestamp sources side-by-side
with per-probe breakdown and aggregate statistics
(min/max/mean/p50/p99/stddev/jitter)

## Diff Breakdown
| Category     | Files | Lines (+/-) | Net  |
|--------------|-------|-------------|------|
| Core logic   |     1 | +783 / -0   | +783 |

Single-file diagnostic tool, no tests or scaffolding.

<details>
<summary>Key files (click to expand)</summary>

- `tools/twamp/cmd/twamp-debug/main.go` — standalone debug tool:
capability probing via real sends, multi-source RTT comparison,
statistics

</details>

## Testing Verification
- Built and deployed to `chi-dn-dzd1`, verified against live TWAMP
reflector on `172.16.0.3:862` over `sw1_11_2`
- Confirmed capability detection correctly identifies TX_SCHED as
delivered, TX_SOFTWARE/TX_HARDWARE/RX_HARDWARE as not delivered despite
setsockopt accepting them
- Verified 200-probe run produces consistent RTT statistics across
userspace, kernel_ns, and software timestamp sources

Author: snormore

CHANGELOG.md

@@ -12,6 +12,8 @@ All notable changes to this project will be documented in this file.
   - Reset SIGPIPE to SIG_DFL at the start of main() in all 3 CLI binaries (doublezero, doublezero-geolocation, doublezero-admin) so the process exits silently like standard CLI tools
 - SDK
   - Add Go SDK for shred subscription program with read-only account deserialization (epoch state, seat assignments, pricing, settlement, validator client rewards), PDA derivation helpers, RPC fetchers, compatibility tests, and a fetch example CLI
+- Tools
+  - Add `twamp-debug` diagnostic tool for testing kernel timestamping support on switches; sends real TWAMP probes to verify which SO_TIMESTAMPING modes (RX/TX software/hardware/sched) actually deliver timestamps, and reports RTT statistics comparing userspace vs kernel timestamp sources
 
 ## [v0.15.0](https://github.com/malbeclabs/doublezero/compare/client/v0.14.0...client/v0.15.0) - 2026-03-27