Anamorphic media: pixel aspect ratio in placement, monitors and New Sequence From Clip

A 1440x1080 clip with 4:3 pixels showed squeezed: the demuxers read its
pixel aspect ratio, but nothing after import used it. New Sequence From
Clip made a 1440x1080 sequence with square pixels, the Motion transform
sized clips from storage pixels (pillarboxed at 1440 wide in a 1920x1080
sequence), and the monitors drew the frame at its storage aspect.

- project: checked_par / sane_par / par_ratio / conformed_size treat
  file and project ratios as hostile (a zero term, or more than 8:1
  either way, is square). MediaClip::pixel_aspect (Interpret Footage
  wins over the file), Project::source_par (subclips follow the parent,
  nested sequences have their own, graphics and adjustment layers have
  none) and Project::conformed_source_size.
- render: motion_matrix takes the source's ratio and works in square
  display units (anchor in source pixels, position in sequence pixels,
  scale and rotation at the display aspect); Scale to Frame Size fits
  the display size. CPU, GPU plan, adjustment regions and the mask /
  graphics overlays share it.
- engine: New Sequence From Clip, New Sequence from an item and
  multicam sequences copy the clip's ratio; default media scaling and
  Fit / Fill frame use the display size; generators made for a
  non-square sequence get its pixels; transcodes render pixel for
  pixel. clip.interpretFootage takes pixelAspect ([num, den] or "file").
- ui: the Source and Program monitors draw the picture at its display
  aspect; overlays map both axes; Sequence Settings shows the display
  aspect (16:9 for 1440x1080 at 4:3).

Tests: render par_tests (fill, GPU plan parity, Scale to Frame Size,
non-square sequences, rotation, Interpret Footage, hostile ratios),
engine par_tests (New Sequence From Clip, overwrite into 1920x1080,
default scaling, Fit / Fill, Interpret Footage with undo and refused
values, hostile file ratios, generators, and ffmpeg-made MP4 and MKV
files), project unit tests, and the headless par_ui monitor test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture.md

@@ -220,6 +220,16 @@ file ──► codecs (MP4/MOV, MKV, audio)        demux + decode, GOP-aware see
   `audio(start, frames, rate)` in media time. Sources are `Send + Sync` and shared by monitors,
   thumbnails, playback and export. The engine's `MediaPool` creates one per project item, lazily,
   through registered openers (`codecs::openers()`: MP4/MOV, MKV/WebM, audio files).
+- **Pixel aspect ratio.** Demuxers report it in `VideoStreamInfo::par` (MP4/MOV `pasp`, Matroska
+  display size, MXF aspect ratio, MPEG sequence header / VUI); Interpret Footage's
+  `Interpretation::par` overrides it (`clip.interpretFootage {pixelAspect}`), and
+  `SequenceSettings::par` is the sequence's. Both go through `project::checked_par` (a zero term
+  or more than 8:1 either way = square). `render::motion_matrix` works in square display units,
+  so a 1440 x 1080 clip with 4:3 pixels fills a 1920 x 1080 square-pixel sequence at 100 %
+  (Scale to Frame Size, Set to Frame Size and Fit / Fill use `Project::conformed_source_size`);
+  graphics and adjustment layers are in sequence pixels. New Sequence From Clip copies the clip's
+  ratio, and the Source and Program monitors draw the picture at its display aspect. The
+  renderer does not read decoded frames' own `VideoFrame::par`.
 - **Offline media and proxies.** The pool caches each item's source per reference (path, offline
   flag), so relinking and undo take effect at once. Media that can't be opened renders the offline
   slate (`render::offline`) instead of failing. With proxies enabled, an item with a proxy reads