MDL writer pipe connector flags may not match Vensim originals #403

Description

In write_flow_pipe_connectors (src/simlin-engine/src/mdl/writer.rs), the pipe connector direction flag (field 4 in the pipe connector tuple) is set to 100 for cloud endpoints and 4 for stock endpoints. However, analysis of the mark2.mdl roundtrip test shows a mismatch between original and roundtripped values:

• Original file: flags {4: 9, 100: 4} (i.e., some stock-connected pipes have flag 9, some have 4; cloud-connected pipes have 100)
• Roundtripped file: flags {4: 11} for stock-connected pipes

The flag semantics appear to involve more than just endpoint type. They may encode additional information such as:

• Flow direction (inflow vs. outflow)
• Source vs. sink role of the connected stock
• Pipe orientation or routing hints

Why It Matters

• Correctness: Roundtripped MDL files may produce visually different pipe/flow rendering in Vensim due to incorrect direction flags.
• MDL format fidelity: Achieving faithful MDL roundtrips is a goal of the current work on the mdl-roundtrip-fidelity branch.
• Interoperability: Users who roundtrip models through Simlin and then open them in Vensim could see unexpected layout changes in flow arrows.

Component(s) Affected

• simlin-engine -- specifically src/simlin-engine/src/mdl/writer.rs, function write_flow_pipe_connectors

Possible Approaches

1. Compare multiple Vensim-generated .mdl files to catalog the full set of flag values and their correlation with flow direction, endpoint type, and pipe orientation.
2. Check Vensim documentation or SDeverywhere/PySD for any reverse-engineered documentation of the pipe connector format.
3. Add test cases that cover inflow-to-stock, outflow-from-stock, and cloud-terminated flows to isolate the flag semantics.

Context

Identified during development on the mdl-roundtrip-fidelity branch, specifically when analyzing differences between the original test/mdl/mark2.mdl and its roundtripped output.