MDL roundtrip loses original view coordinates due to compose_views offset

[bpowers]

Problem

When multiple MDL views are parsed, compose_views (in src/simlin-engine/src/mdl/view/processing.rs) stacks views vertically starting at (100, 120), applying an additive coordinate offset. This offset is NOT reversed during MDL writing. The roundtripped coordinates differ from the originals by a constant offset per view (e.g., +18 for x, +112 for y in view 1 of mark2.mdl).

This does not affect simulation correctness, but will confuse Vensim users who open a roundtripped file and find their diagram layout shifted.

Components affected

• src/simlin-engine/src/mdl/view/processing.rs (compose_views function)
• src/simlin-engine/src/mdl/writer.rs (MDL serialization path)

Why it matters

• User experience: Vensim users expect opening a roundtripped MDL file to show the same layout they saved. Shifted coordinates break that expectation.
• Roundtrip fidelity: MDL roundtrip tests currently must accept coordinate offsets as known divergences rather than asserting exact equality.

Possible approaches

1. Store original coordinates in ViewElementCompat -- preserve the pre-transformation coordinates alongside the composed ones, and use the originals when writing back to MDL.
2. Skip coordinate transformation for MDL-sourced models -- avoid applying the compose_views offset when the source format is MDL, since Vensim handles multi-view layout itself.
3. Reverse the transformation during writing -- compute the view segment's min_x/min_y and subtract the offset when serializing back to MDL.

Context

Identified during MDL roundtrip fidelity work on the mdl-roundtrip-fidelity branch. The mark2.mdl test model demonstrates the issue clearly with its multiple views.