chore(paged): add current serving snapshot harness

Add a reusable current-stack paged-vs-vLLM serving snapshot harness that targets the clean DGX mirror, enforces idle/lock preflight, runs pre/post inference gates, and records ratio summaries.

Assisted-by: Codex:gpt-5

docs/superpowers/plans/2026-07-01-current-serving-harness-phase21.md

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+# Current Serving Harness Phase 21 Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use
+> superpowers:verification-before-completion before recording the phase result.
+> Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** make the Phase 20 current-stack paged-vs-vLLM serving snapshot
+repeatable from the LocalAI backend tree.
+
+**Architecture:** add a standalone shell harness beside the existing paged
+inference gate and MTP serving harness. The script targets the clean
+`~/llama-phase6-source` mirror, uses the owner-file GPU lock, runs pre/post
+inference gates, compares paged and vLLM in one session, and writes ratio
+summaries.
+
+**Tech Stack:** Bash, llama.cpp `llama-server`, vLLM, `h2h_cli3.py`, DGX GB10.
+
+---
+
+## Task 1: Red Check
+
+- [x] **Step 1: Prove no reusable current-stack harness exists**
+
+  Command:
+
+  ```bash
+  test -e backend/cpp/llama-cpp-localai-paged/paged-current-serving-snapshot.sh
+  ```
+
+  Result:
+
+  - exited `1` before the patch, as expected.
+
+## Task 2: Add Harness
+
+- [x] **Step 1: Create script**
+
+  File:
+
+  - `backend/cpp/llama-cpp-localai-paged/paged-current-serving-snapshot.sh`
+
+  Features:
+
+  - defaults to `~/llama-phase6-source`, not stale `~/llama-paged-dev`;
+  - checks docker, `local-ai-worker`, GPU compute processes, and owner-file lock;
+  - builds `llama-server`, `llama-completion`, and `test-backend-ops`;
+  - runs pre/post `paged-inference-gates.sh`;
+  - runs paged and vLLM serving arms with the same h2h client;
+  - writes `summary.tsv` with paged/vLLM ratios;
+  - supports `DRY_RUN=1` for path/preflight validation without servers.
+
+## Task 3: Verify Harness
+
+- [x] **Step 1: Local syntax/help checks**
+
+  Commands:
+
+  ```bash
+  test -x backend/cpp/llama-cpp-localai-paged/paged-current-serving-snapshot.sh
+  bash -n backend/cpp/llama-cpp-localai-paged/paged-current-serving-snapshot.sh
+  backend/cpp/llama-cpp-localai-paged/paged-current-serving-snapshot.sh --help
+  ```
+
+  Result:
+
+  - all passed.
+
+- [x] **Step 2: DGX dry run**
+
+  Command:
+
+  ```bash
+  DRY_RUN=1 ART=~/bench/phase21_harness_dryrun/20260701_051757 \
+    /tmp/paged-current-serving-snapshot.sh
+  ```
+
+  Result:
+
+  - verified `docker=0`, `local_ai_worker=0`, `compute=0`;
+  - verified owner file was free;
+  - found current source `f2521ab12`;
+  - validated required paths and printed the build/paged/vLLM commands without
+    launching servers.
+
+  Artifact:
+
+  - `/home/mudler/bench/phase21_harness_dryrun/20260701_051757`
+
+## Task 4: Future Use
+
+- [x] **Step 1: Prefer this harness for current snapshots**
+
+  Use this script for future current-stack GB10 parity snapshots:
+
+  ```bash
+  backend/cpp/llama-cpp-localai-paged/paged-current-serving-snapshot.sh
+  ```
+
+  Do not use the stale DGX `~/bench/combined_definitive.sh` without first
+  porting it to the clean mirror and owner-file lock discipline.
+
+## Self-Review
+
+- No llama.cpp source behavior changed.
+- The harness is repeatable and defaults to the current clean mirror.
+- The dry run covered path validation and DGX preflight without consuming GPU
+  benchmark time.