[crisprintsstuff]

Fixes #234: the Measure plugin now shows units.

What changes

• Preferences > General > Units (next to Orbit around):
  • Metric (mm) or Imperial (in).
  • 1 scene unit = N mm. This is the 3D print window's existing scale (default 10), now saved between sessions, so the two always agree.
• Plugin API 5:
  • units() -> i32: 0 metric, 1 imperial.
  • unit_scale() -> f32: millimetres per scene unit, never 0.
  • Older plugins are unaffected. The API reference and manifest docs are updated.
• Measure 1.1 (now API 5):
  • Metric: "Size 20.000 × 10.000 × 5.000 mm, volume 1.000 cm³".
  • Imperial: "… in, volume … in³".
  • Its number formatting now uses 64-bit integers, so a metre-sized model in millimetres doesn't overflow (before, values above about 4.29 million thousandths saturated).

Verified

• cargo fmt --check, clippy -D warnings, cargo test --workspace, check_corpus_ip.py.
• New and updated tests:
  • Measure output in metric, imperial, and for a large model.
  • Any plugin can read units(), and unit_scale() falls back to 1 for a zero scale.
  • The settings round trip carries the units and the scale.
  • The manifest test now refuses API 6.
• A headless screenshot opened Preferences without errors. The Units row sits further down the General page, below the part captured, so I didn't see it rendered.

Not done

• The 3D print window still shows millimetres whatever the Units choice, since slicers work in mm. The scale itself is shared.
• Not run on Windows or macOS.

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Generated by Claude Code