add Modal noise-regen sync-back entrypoint

`make measure-noise` runs on the local CPU, but the canonical drift check
(tests/docs/test_numerical_noise_drift.py under `make test`) runs on Modal's
CPU, where a few boundary measurements round differently. This entrypoint runs
scripts.measure_op_noise on a Modal worker matching the test shard's container
and syncs the regenerated docs/footers back locally — the CLAUDE.md
artefact-sync-back case, importing IMAGE from modal_image.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: physicsrob

modal_measure_noise.py

@@ -0,0 +1,58 @@
+"""Regenerate the op-noise artefacts on Modal's CPU and sync them back locally.
+
+`make measure-noise` runs on the local CPU, but the canonical drift check
+(`tests/docs/test_numerical_noise_drift.py` under `make test`) runs on Modal's
+CPU, and a few measurements sit on precision boundaries where local-CPU and
+Modal-CPU round differently. This entrypoint runs `scripts.measure_op_noise` on a
+Modal CPU worker (the same environment as the drift check) and returns the
+regenerated `docs/op_noise_data.json`, `docs/numerical_noise.md`, and every
+`torchwright/ops/*.py` (for the in-place noise-footer edits) so the local tree
+matches what `make test` will measure.
+
+This is the artefact-sync-back case the CLAUDE.md "Running scripts on GPU" rules
+carve out as the only acceptable reason for a purpose-built `modal_*.py` — and it
+imports `IMAGE` from `modal_image` rather than duplicating it.
+
+    uv run modal run modal_measure_noise.py
+"""
+
+import pathlib
+import subprocess
+import sys
+
+import modal
+
+from modal_image import IMAGE
+
+app = modal.App("torchwright-measure-noise", image=IMAGE)
+
+
+# Match the test shard's container exactly (modal_test.py: gpu a100-80gb, cpu 8,
+# memory 32768). `_measure_all` forces CPU compute, but reproducing the same
+# container is what makes a few precision-boundary measurements (e.g. reciprocal
+# p99) match what the drift check under `make test` measures.
+@app.function(gpu="a100-80gb", cpu=8, memory=32768, timeout=1800)
+def measure() -> dict:
+    rc = subprocess.run(
+        [sys.executable, "-m", "scripts.measure_op_noise"]
+    ).returncode
+    if rc != 0:
+        raise RuntimeError(f"scripts.measure_op_noise exited {rc}")
+
+    from scripts.measure_op_noise import DOCS_JSON, DOCS_MD, REPO_ROOT
+
+    out: dict[str, str] = {}
+    for p in (DOCS_JSON, DOCS_MD):
+        out[str(p.relative_to(REPO_ROOT))] = p.read_text()
+    for f in sorted((REPO_ROOT / "torchwright" / "ops").glob("*.py")):
+        out[str(f.relative_to(REPO_ROOT))] = f.read_text()
+    return out
+
+
+@app.local_entrypoint()
+def main():
+    files = measure.remote()
+    root = pathlib.Path(__file__).resolve().parent
+    for rel, content in sorted(files.items()):
+        (root / rel).write_text(content)
+        print(f"wrote {rel} ({len(content)} bytes)")