Extract _target_size helper + regression-test native resolution (v0.5.4)

The native-vs-downscale decision in InvisibleEngine.remove_watermark (the
issue #10/#15 fix: max_resolution=0 must not pre-downscale, since any
downscale both loses quality and lets SynthID survive) had no test. Extract
it into a pure helper invisible_engine._target_size(w, h, max_resolution)
and cover it with tests/test_invisible_engine.py::TestTargetSize so a
re-introduced forced downscale fails CI instead of silently regressing #15.

Also:
- Clamp the short side to >=1 in _target_size: extreme aspect ratios (e.g.
  5000x3 with --max-resolution 1024) truncated it to 0 and crashed
  image.resize(). Pre-existing in the inline math; fixed now that it is a
  named, tested function.
- Consolidate the two duplicated temp-file save blocks into one
  unconditional save (behavior unchanged: the EXIF-transposed image is
  still always persisted before WatermarkRemover reloads it by path), and
  drop the now-redundant `_tmp_path is not None` guard in finally.
- Bump version 0.5.3 -> 0.5.4 (pyproject, __init__, uv.lock); document the
  helper as the regression guard in CLAUDE.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "remove-ai-watermarks"
-version = "0.5.3"
+version = "0.5.4"
 description = "Remove visible and invisible AI watermarks from images (Gemini / Nano Banana, ChatGPT, Stable Diffusion)"
 readme = "README.md"
 requires-python = ">=3.10"