Add OpenArt visible watermark detection and removal

OpenArt.ai stamps free-plan generations with a semi-transparent white
"OpenArt" wordmark (bowtie/infinity icon + brand name) placed near the
CENTER of the frame rather than in a corner. Confirmed from a customer-
reported raiw-app production case where the mark went completely
undetected and unremoved, with no C2PA/EXIF/TC260 signal for identify
to fall back on.

- Add a new "cc" (frame-center) corner mode to the shared
  TextMarkConfig/TextMarkEngine base, alongside the existing
  corner/edge anchors.
- Add openart_engine.py, following the existing text-mark engine
  pattern. No real capture exists (OpenArt requires an account, out of
  scope to create), so the bundled alpha asset is a procedural
  reconstruction of the public logo and the NCC threshold is an
  explicitly-labeled provisional guess, not a measured calibration.
- Register the mark in watermark_registry.py, which auto-derives
  identify's platform attribution.
- Add tests/test_openart_engine.py (synthetic composites only) plus
  registry/identify test coverage, and document the mark and its
  calibration caveats in README.md, docs/supported-signals.md,
  docs/module-internals.md, and the skill's commands.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/build_openart_alpha.py

@@ -0,0 +1,93 @@
+"""Build the PROVISIONAL synthetic OpenArt alpha asset.
+
+Unlike ``visible_alpha_solve.py``, this does NOT recover an alpha map from a
+controlled capture: OpenArt (openart.ai) requires an account to generate
+images (no anonymous or API-key access, confirmed 2026-09-21 via its own
+help center and MCP docs), and creating an account is outside what this
+project's tooling is allowed to do on its own. No committed capture exists
+under ``data/calibration/openart/``.
+
+This script instead PROCEDURALLY DRAWS a reconstruction of OpenArt's own
+brand mark (the bowtie/infinity icon followed by the "OpenArt" wordmark, as
+shown on openart.ai's own marketing pages) as a semi-transparent white
+overlay, matching the general rendering style (near-white, low-saturation,
+anti-aliased) that every other bundled text-mark asset in this module was
+solved from. It is a reconstruction from the public logo, not a recovered
+watermark alpha map, and ``openart_engine.py`` documents that provenance
+gap explicitly. Replace this asset (and re-run the engine's calibration)
+the moment a real captured OpenArt export is available under
+``data/calibration/openart/``.
+
+Usage::
+
+    uv run python scripts/build_openart_alpha.py
+"""
+
+# cv2/numpy boundary: third-party libs ship no usable element types; relax the
+# unknown-type rules for this file only (mirrors visible_alpha_solve.py).
+# pyright: reportUnknownMemberType=false, reportUnknownArgumentType=false, reportUnknownVariableType=false, reportUnknownParameterType=false, reportMissingTypeArgument=false, reportMissingTypeStubs=false, reportMissingImports=false, reportArgumentType=false, reportAssignmentType=false, reportReturnType=false, reportCallIssue=false, reportIndexIssue=false, reportOperatorIssue=false
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+import cv2
+import numpy as np
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+_ROOT = Path(__file__).resolve().parents[1]
+_ASSET = _ROOT / "src" / "remove_ai_watermarks" / "assets" / "openart_alpha.png"
+
+# Canvas sized to comfortably fit "OpenArt" set bold at this height plus the
+# icon and inter-glyph gap, at a resolution close to the other bundled
+# assets (doubao_alpha.png is 335x83).
+_W, _H = 360, 90
+_ALPHA_PEAK = 173  # matches doubao_alpha.png's measured peak (~68% opacity)
+
+
+def _draw_infinity_icon(canvas: np.ndarray, cx: int, cy: int, size: int) -> None:
+    """Draw the bowtie/infinity glyph left of the wordmark (two mirrored lobes)."""
+    half = size // 2
+    # Two overlapping filled triangles meeting at the center, echoing the
+    # angular "infinity" mark on openart.ai's own site header.
+    left_lobe = np.array(
+        [[cx - size, cy - half], [cx - size, cy + half], [cx, cy]],
+        dtype=np.int32,
+    )
+    right_lobe = np.array(
+        [[cx + size, cy - half], [cx + size, cy + half], [cx, cy]],
+        dtype=np.int32,
+    )
+    cv2.fillConvexPoly(canvas, left_lobe, (_ALPHA_PEAK,), lineType=cv2.LINE_AA)
+    cv2.fillConvexPoly(canvas, right_lobe, (_ALPHA_PEAK,), lineType=cv2.LINE_AA)
+
+
+def build() -> np.ndarray:
+    canvas = np.zeros((_H, _W), np.uint8)
+    icon_cx, icon_cy, icon_size = 34, _H // 2, 22
+    _draw_infinity_icon(canvas, icon_cx, icon_cy, icon_size)
+
+    text_x = icon_cx + icon_size + 18
+    cv2.putText(
+        canvas,
+        "OpenArt",
+        (text_x, _H // 2 + 16),
+        cv2.FONT_HERSHEY_DUPLEX,
+        1.3,
+        (_ALPHA_PEAK,),
+        thickness=2,
+        lineType=cv2.LINE_AA,
+    )
+    return canvas
+
+
+def main() -> None:
+    canvas = build()
+    _ASSET.parent.mkdir(parents=True, exist_ok=True)
+    cv2.imwrite(str(_ASSET), canvas)
+    print(f"Wrote {_ASSET} ({canvas.shape[1]}x{canvas.shape[0]}, peak={int(canvas.max())})")
+
+
+if __name__ == "__main__":
+    main()