tests : add fusion baseline README and broaden fusion CI triggers (#28893)

* tests : add README for updating the per-backend fusion baselines

Assisted-by: pi:llama.cpp/Qwen3.8-27B

* ci : trigger fusion on changes to test-llama-archs.cpp and src/models

the dummy models and their architectures drive the fusion baselines, so a
change to either can alter the per-fusion counters and should re-run the
fusion job.

Assisted-by: pi:llama.cpp/Qwen3.8-27B

* tests : merge the fusion build commands in the README

assisted-by: pi:llama.cpp/Qwen3.8-27B

* pi : require explicit permission before posting PR/issue comments

assisted-by: pi:llama.cpp/Qwen3.8-27B

tests/fusion/README.md

@@ -0,0 +1,26 @@
+# Fusion baselines
+
+Per-device baselines for `test-fusion`, one CSV per backend (e.g. `MTL.csv`). Rows are
+`arch,moe,mode,label,count`. Regenerate a CSV whenever fusion patterns change.
+
+## Update a baseline
+
+```sh
+cmake -B build -DCMAKE_BUILD_TYPE=Release -DGGML_METAL=ON   # enable the target backend
+cmake --build build --config Release --target test-llama-archs --target test-fusion -j
+
+rm -rf build-ci-models && mkdir -p build-ci-models
+./build/bin/test-llama-archs -o build-ci-models
+
+./build/bin/test-fusion --models build-ci-models --device MTL0 --record MTL.csv
+```
+
+## Validate
+
+```sh
+./build/bin/test-fusion --models build-ci-models --device MTL0 --check MTL.csv
+```
+
+Non-zero exit means a row differs from the baseline. Use `--model FILE` to run a single
+architecture. Note `--check` only sees present rows — a fusion that stops matching is not
+reported, so diff the recorded CSV to catch removed patterns.