docs(char-rp-gguf): record custom llama.cpp build (master 6eddde0 + unmerged PR #25544)

Durable record of the custom llama.cpp the char-rp-reasoning seat will run to
fix Worldtree #355 (reasoning-budget forcing broken in stock b8840 — single
end-tag </think> can't match Qwen3.5's <tool_call> reasoning terminator, so the
budget never force-closes and reasoning runs away to max_tokens). PR #25544
adds multiple terminating sequences; unmerged upstream, so we build it.

- build.sh: reproducible recipe (clone master@6eddde0 + merge PR #25544 +
  resolve the 1 server-common.cpp conflict + CUDA build for Blackwell sm_120)
- README.md: why + acceptance test + rollback + REMOVE-WHEN-MERGED tracking

Image llamacpp-charrp:6eddde0-pr25544 BUILT + smoke-tested on ana-ml2; seat
swap pending. See also auto-memory reference_charrp_custom_llamacpp_pr25544.

stacks/char-rp-gguf/llamacpp-custom/README.md

@@ -0,0 +1,100 @@
+# ⚠️ CUSTOM llama.cpp build for the char-rp-reasoning seat
+
+> **STATUS (2026-07-13):** image `llamacpp-charrp:6eddde0-pr25544` (+ `:custom-latest`)
+> BUILT on ana-ml2 (reports `version: 10001 (6ce128d0b)`, CUDA 12.8.1, sm_120) and
+> binary-smoke-tested OK. **Seat swap PENDING** — the live `char-rp-gguf` compose
+> still points the reasoning seat at stock 8840 until the swap + on-seat forced-close
+> validation. Once swapped, update this line to DEPLOYED.
+
+**The `llama-charrp-reasoning` seat (Deckard-PKD, ana-ml2:8018) runs a
+CUSTOM-BUILT llama.cpp, NOT a stock upstream image.** If you are upgrading,
+rebuilding, or "cleaning up" the seat, read this first — a naive bump to a
+stock image will silently reintroduce Worldtree #355.
+
+## What it is
+
+`llamacpp-charrp:custom-latest` = **llama.cpp master `6eddde0`** (2026-07-13,
+~b9990 era) **+ unmerged upstream PR [#25544]** (`aldehir:reasoning-budget-multi-seq`),
+built for Blackwell (sm_120) via `build.sh` in this directory.
+
+Merged tree HEAD (on ana-ml2 `/home/lkraven/llamacpp-build`): `6ce128d0b`.
+
+[#25544]: https://github.com/ggml-org/llama.cpp/pull/25544
+
+## Why it exists (Worldtree #355 root cause)
+
+Stock llama.cpp **b8840** (the previous seat build, `ghcr.io/mostlygeek/llama-swap:cuda`)
+— and every *released* build to date — has a reasoning-budget sampler that
+recognises only **one** end-of-thinking tag, `</think>`. But Qwen3.5's
+tool-call path ends its reasoning block with **`<tool_call>`**, a terminator
+the single-tag sampler can't match. So on the agentic tool-retry path,
+`--reasoning-budget 400`'s forced-close **never fires**, and the model loops
+in reasoning all the way to `max_tokens` (32768) ≈ **22 min per turn** (~13%
+of char-rp-reasoning turns). That is #355's residual hang.
+
+Evidence that pinned it (all three agree): LiteLLM spend_logs (4 requests at
+exactly `completion_tokens=32768`), the seat's own log (`158 activated / 0
+forced-close / 110 natural-end`), and the WT↔gateway pcap (runaway stream is
+100 % `reasoning_content`, coherent tool-retry planning, never exits `<think>`).
+
+**PR #25544 is the fix**: it extends the budget sampler to **multiple
+terminating sequences** (`</think>` *or* `<tool_call>`) and replays the matched
+end-sequence when the budget expires. It is **OPEN / unmerged** upstream
+(the underlying trigger #22684 is closed-as-not-planned; the tool-parsing PR
+#24202 is also open), so there is **no released build that fixes our bug** —
+hence this local build.
+
+## Conflict resolution (the one thing to re-verify on any rebump)
+
+The PR (based on master of 2026-07-10) is 25 commits behind `6eddde0`; the
+merge has a single conflict in `tools/server/server-common.cpp`. Resolution
+(baked into `build.sh`): keep the **PR's plural `reasoning_budget_end_tags`**
+(the fix) **and master's per-request body-read of `reasoning_budget_message`**
+(a newer master feature). Both improvements retained.
+
+## Build / rebuild
+
+```bash
+# on ana-ml2 (has the Blackwell GPUs + docker + disk):
+scp stacks/char-rp-gguf/llamacpp-custom/build.sh ana-ml2:/home/lkraven/
+ssh ana-ml2 'bash /home/lkraven/build-charrp.sh'   # ~20-40 min, sm_120 compile
+```
+
+Then deploy via the seat compose (`stacks/char-rp-gguf/compose.yaml`, service
+`llama-charrp-reasoning`), whose `image:` is pinned to `llamacpp-charrp:custom-latest`.
+
+## Acceptance test before trusting a build (do NOT skip)
+
+The bug is specifically that forcing *never fires*. A build is only good if it
+demonstrably **force-closes** — not just "loads + serves":
+
+1. `tests/test-reasoning-budget.cpp` passes (`ctest -R reasoning-budget`).
+2. Live low-budget check on Deckard: launch with `--reasoning-budget 20`, send a
+   tools-bearing request that induces a tool-retry loop, and confirm the seat
+   logs a **forced** close (budget-reached), not just `deactivated (natural end)`,
+   and the response stops near the budget instead of running to `max_tokens`.
+
+Then restore `--reasoning-budget 400` for production.
+
+## Rollback
+
+The previous stock image `ghcr.io/mostlygeek/llama-swap:cuda` (build 8840) is
+preserved on ana-ml2 (imageID `d6c39f55…`). To roll back: point the seat's
+`image:` back to it and recreate. 8840 works (RP is fine) — it only has the
+forcing bug.
+
+## 🔔 Retiring this custom build (REMOVE-WHEN-MERGED)
+
+This is a temporary local build carrying an unmerged PR. **When PR #25544
+merges upstream and lands in a stock release:**
+
+1. Confirm the released build's `tests/test-reasoning-budget.cpp` covers the
+   `<tool_call>` terminator and passes on Qwen3.5.
+2. Switch the seat `image:` to that stock image (e.g. a
+   `ghcr.io/ggml-org/llama.cpp:server-cuda` tag ≥ the merge) and run the
+   acceptance test above.
+3. Delete this directory + the compose `⚠️ CUSTOM BUILD` note, and update
+   `persistent-memory.md` + the `reference_charrp_custom_llamacpp_pr25544`
+   auto-memory.
+
+Check status: <https://github.com/ggml-org/llama.cpp/pull/25544>.