fix(image): charge GIF frames at the canvas size Pillow decodes them at

[badGarnet]

Summary

Follow-ups to the image pixel limit added in #4520.

• Charge each GIF frame at the canvas size Pillow decodes it at. A GIF frame that extends past the logical screen grows the canvas that it and every later frame are decoded onto. Pillow's frame count (n_frames) does not apply that growth, so the limit charged every frame at the first frame's size, and a small first frame followed by a large one was undercounted.
  • With hi_res, a GIF's frames are now measured by _iter_gif_canvas_sizes(), which reads each frame descriptor's position and size from the file.
  • It skips color tables and image data by their lengths, and stray bytes as Pillow does, without decompressing anything, so the work is linear in the file size.
  • It yields the running canvas size: the logical screen grown to fit every descriptor seen so far.
  • Other formats are unchanged.

Verification

• Exact match: on 2,000 generated GIFs with random logical screens, frame positions and sizes, extensions and stray bytes, the walker's per-frame canvas sizes equal the sizes Pillow decodes each frame at. This was run in a memory-capped container.
• Never fewer pixels: on 329 byte-mutated GIFs that Pillow could still decode, the walker never charged fewer pixels than Pillow decoded.

Tests

• GIF walker: matches Pillow's decoded canvas on a fixed canvas, growth on a later frame, growth by offset, and a first frame past the screen (with an extension and a stray byte).
• Growing canvas: a GIF whose canvas grows from 10×10 to 100×80 is admitted at its exact decoded total and rejected one below it, allocating no pixel buffer.
• Frame-header failure: a failure after one frame was measured, through partition_image. No partitioner, OCR or conversion runs, the original exception escapes, and the borrowed stream stays open at its position.
• ocr_only negative controls: whether requested directly or as hi_res's fallback when unstructured_inference is missing, ocr_only never reaches a later frame or a declared frame count, for TIFF, APNG and GIF. A hi_res control shows the same poison trips when frames are read.
• Forged frame count: a deterministic work count for a forged APNG acTL: one run, not one per declared frame.

🤖 Generated with Claude Code

[cubic-dev-ai]

All reported issues were addressed across 4 files

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