tt-model authoring files (fused default, megakernel pass)

SERVING.md

@@ -218,3 +218,23 @@ once per grid).
    `to_tensor`/`normalize` in torch (bit-identical, checked offline).
 8. The tree venv is Python 3.10; the image is 3.12 (uv dry-run of the exact package set on 3.12
    resolves to torch 2.11.0+cpu / numpy 1.26.4 / transformers 5.12.1, no torchvision).
+9. **`TT_FUSED` fused device paths (default ON since the 2026-09-13 device validation).**
+   Branch `opt/locate-anything-3b-p150-megakernel` adds fused device paths behind one env knob
+   read once at startup (`code/locate_anything/tt/fused.py`): the MoonViT graph as one metal
+   trace with the patch merger on device and a row-major pixel upload, the vision->LLM embedding
+   merge on device, a traced 36-layer prefill with an eager norm/LM-head tail, a one-row
+   first-token readback and on-device greedy sampling in the decode loop. The default fused
+   configuration keeps the legacy vision numerics bit for bit (device-verified) and measured
+   360.9 -> 304.9 ms per demo request in the host pipeline (see `DEVICE_VALIDATION.md`
+   "Results" for the served numbers). `TT_FUSED=0` = the 2026-09-12 shipped device graph (same
+   ops, same order, same arguments, same numerics); the only host-visible additions are the
+   `"fused"` key in the pipeline `run()` result dict and the `fused` entry of `GET /info` -- the
+   `/predict` body is unchanged. With the path on, `LA_TRACE_REGION_SIZE` defaults to 160000000
+   and the warm-up must capture the vision and prefill traces before READY (startup error
+   otherwise); `/info` reports `fused.config` / `fused.status` and `timing_ms.vision` becomes
+   the non-blocking trace submit (compare `total`). Measured opt-ins that change the bf16
+   rounding of the vision tower (not default; numbers in `DEVICE_VALIDATION.md`):
+   `LA_FUSED_EXACT_SEQ=1 LA_FUSED_SDPA_CHUNKS=96,352 LA_FUSED_SDPA_EXP_APPROX=0` (another
+   -13 ms; the demo box becomes the HF reference's `<282><414><607><797>` instead of the shipped
+   `<606><794>`) and `LA_FUSED_MATMUL=minimal` (below the 0.99 PCC gate on the port golden).
+   Host tests: `code/locate_anything/tests/test_fused_host.py`.