docs(#662): Phase 2 bench results — Go PoC proven on arm64 (CA-compat/204/inject/JA4/12MB); throughput gate deferred to controlled bench

docs/superpowers/specs/2026-06-18-mitm-engine-phase2-bench.md

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+# MITM engine migration — Phase 2 bench results (#662)
+
+- **Date:** 2026-06-18 · ran the Phase-1 Go PoC on gk2 (arm64), `127.0.0.1:8090`,
+  **no DNAT** (zero impact on live R3, which stayed on the mitmproxy workers).
+
+## Proven on the real arm64 board (with the live `ca-wg` CA)
+| Check | Result |
+|-------|--------|
+| Static arm64 binary | 5.4 MB, `ELF aarch64`, CGO_ENABLED=0 — runs natively on gk2 |
+| **CA-compat forging** | `curl -x :8090 --cacert ca.pem https://example.com/` → **200**; the forged leaf (signed by the existing `ca-wg` CA) is trusted — R3 clients would trust it, no re-enroll |
+| **MITM + body inject** | injected `<!-- sbx-ng banner -->` marker present in the HTML |
+| **204 block** | `https://doubleclick.net/` → **204** (ad_ghost path) |
+| **JA4 capture** | live: `t0304_c31_ah2_sni=example.com` (TLS1.3 / 31 ciphers / ALPN h2 / SNI) — the `ja4` addon's material is reachable in Go on arm64 |
+| **Footprint** | **~12 MB RSS** vs Python mitmproxy ~70–117 MB per worker |
+
+So every **discriminating capability** the analysis flagged (CA-compat, request-204,
+body-inject, SNI-splice, JA4) works on the actual hardware, at ~1/6th the memory.
+
+## Gate: "forge + throughput ≥ mitmproxy" — PARTIAL
+- **Forge:** ✅ proven (CA-compat, cached per host, fast).
+- **Footprint:** ✅ ~12 MB (far below mitmproxy).
+- **Throughput / multi-core:** ⚠️ **not cleanly measured.** The instantaneous-CPU
+  sample was cut short by (a) a transient `wg-admin` ssh blip and (b) a
+  `pkill -f sbxmitm` self-match bug (the kill matched its own ssh shell). Multi-core
+  is **structurally guaranteed** — Go runs `GOMAXPROCS=4` with no GIL, vs Python
+  mitmproxy capped ~1 core/worker — but a rigorous throughput-vs-mitmproxy
+  comparison must be done in a **controlled load environment**, NOT by hammering
+  the production board.
+
+## Conclusion
+Phase 2 **de-risks the engine on real hardware**: CA-compat + all L7/TLS
+discriminators + tiny footprint confirmed on arm64. The throughput gate is
+deferred to a controlled bench (Phase 2b) before committing to the port.
+
+## Ops note
+The PoC was localhost-only (`127.0.0.1:8090`), no DNAT, cleaned up (`fuser -k
+8090/tcp` + binary removed). LESSON: never `pkill -f <name>` over ssh when `<name>`
+appears in the remote command line — it kills its own shell; use `fuser -k
+<port>/tcp` or `pgrep | grep -v $$` + kill-by-PID.
+
+## Next
+Phase 2b (controlled throughput bench) → Phase 3 (hot-path feature parity). Pause
+for review before continuing — see the phased plan.