Give the stroke pattern one mode and design.yaml tuning

The twelve experimental modes collapse to camera_compensated_width, whose ten
knobs now come from a `stroke_pattern` block, and the PNG-only binary-preview
command goes with them as a strict subset of stroke-texture.

Verified: output is byte-identical at the defaults against the previous code, a
knob change is visible in the artifacts, the suite is green and the doc gate is
clean.

tests/test_stroke_texture_cli.py

@@ -55,7 +55,9 @@ def test_stroke_cli_exports_three_formats_and_manifest(tmp_path, monkeypatch):
 
     def run_blender(script, blend, args):
         calls.append((script, blend, args))
-        output_dir.mkdir()
+        # exist_ok: the CLI writes _stroke_pattern_params.json into the output directory before
+        # it launches Blender, so the directory already exists.
+        output_dir.mkdir(parents=True, exist_ok=True)
         for fmt in ("png", "svg", "pdf"):
             (output_dir / f"stroke_texture_compute.{fmt}").write_bytes(fmt.encode())
         return 0
@@ -63,7 +65,13 @@ def test_stroke_cli_exports_three_formats_and_manifest(tmp_path, monkeypatch):
     monkeypatch.setattr(cli, "_run_blender_script", run_blender)
     assert cli.cmd_stroke_texture(REVISION, _args(job, output_dir)) == 0
     assert calls[0][0] == "random_stroke_pattern.py"
-    assert calls[0][2][-4:] == ["--seed", "13", "--camera-width-fraction", "0.5"]
+    # `--seed` and `--camera-width-fraction` are overrides folded into the params file, so the
+    # contract is now the effective tuning the script reads.
+    params = json.loads(
+        Path(calls[0][2][calls[0][2].index("--params") + 1]).read_text(encoding="utf-8")
+    )
+    assert params["seed"] == 13
+    assert params["width_fraction"] == 0.5
     manifest = json.loads((output_dir / "stroke_texture_manifest.json").read_text())
     assert manifest["schema"] == "orisys-sensor-design/stroke-texture-manifest-v1"
     assert manifest["source_job"]["sha256"] == sha256_file(job)