MDL roundtrip loses per-view font metadata in multi-view models

[bpowers]

Description

During MDL parsing, when multiple views are merged into a single StockFlow, font metadata is collapsed to a single value by taking only the first non-None font (src/simlin-engine/src/mdl/view/convert.rs:262-264). When the merged view is later split back into multiple views for MDL output, all segments share this single font string.

This means multi-view MDL files where different views specify different $... font lines will roundtrip with the wrong font on later views.

Why it matters

• Correctness: roundtripping a multi-view MDL file silently loses information, which could affect downstream tools or users who rely on per-view font settings.
• Maintainability: the current data model (StockFlow has a single font: Option<String>) conflates view-level metadata with model-level metadata, making it harder to reason about what belongs where.

Components affected

• src/simlin-engine/src/mdl/view/convert.rs (merge logic at lines 262-264)
• src/simlin-engine/src/mdl/view/ (output/split logic)
• project_io.proto / data model (StockFlow.font field)

Possible approaches

The fix would require extending the data model so that font metadata is stored per-view rather than per-StockFlow. One approach: attach font metadata to the Group elements that serve as view merge markers, since those already delineate view boundaries within a merged StockFlow. This would require changes to the protobuf schema and both the MDL reader and writer.

Context

Identified during code review of PR #397 (mdl-roundtrip-fidelity branch). The fix requires a data model refactor and was deemed out of scope for that PR.