Fix inch-unit conversion for coordinates and extrusion

[remcoder]

Problem

Fixes #497. Inch-based G-code rendered 25.4 times too small and reported incorrect filament lengths.

Cause and fix

G20 changed the units flag but handlers used command values directly. Distance words now become millimeters before coordinate shifts or extrusion deltas are calculated: distance_mm = distance_in × 25.4. Omitted words preserve existing state; G20/G21 only change interpretation of subsequent commands. G92 uses the same conversion, so its logical coordinates and stored physical offsets agree.

Arc endpoints, I/J offsets and R are normalized together. The tessellator now receives millimeters exclusively, so its chord tolerance uses the physical radius directly. Probe targets use the same conversion and retain Z0 contact behavior. Slicer width/height comments remain millimeters.

Behavior changes

Inch files now produce correctly sized paths, bounding boxes and extrusion lengths. Millimeter files retain their existing behavior. State position documentation specifies millimeters; the internal ArcTessellator no longer accepts a units argument.

Checks

• 992 tests passed; 100% statement, branch, function and line coverage.
• Typecheck, lint and production build passed.
• Pipeline regressions cover M82/M83, E-only moves, G92, unit switches, I/J and R helical arcs (including equal geometry and segment counts), probes, and slicer dimensions.
• Straight-move equivalence now checks geometry, path types, bounds, and filament length in both M82/M83. Inch E-only retract/prime pairs check each intermediate E position, unchanged geometry, and correct subsequent extrusion without a G92 reset.
• All four new regression cases fail when inch extrusion conversion is temporarily removed and pass with the fix restored.
• Browser verification left to the maintainer as requested.

Assisted by OpenAI Codex (GPT-6).

[github-actions]

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[sophiedeziel]

I like that everything is normalized early.

I worry that bugs are easily introduced by forgetting to normalize units whenever a new command is introduced, but that's not in scope of this PR.

Let's think a little bit about the safeguards to add.