Fix MATLAB in-out parameter semantics when input and output share a name

[nallana]

Motivation

• Function lowering currently defines outputs after parameters in the same scope which overwrites parameter bindings when an output shares an input name, breaking MATLAB in-out semantics.
• This causes body references to resolve to the output slot instead of the incoming argument, producing incorrect runtime behavior for common authoring patterns.

Description

• Preserve the parameter binding when an output name matches an existing parameter by adding fn lookup_current_scope(&self, name: &str) -> Option<VarId> and reusing that VarId instead of defining a new one during AstStmt::Function lowering in crates/runmat-hir/src/lib.rs.
• Added a HIR-level regression test function_output_reuses_param_binding_when_names_match in crates/runmat-hir/tests/coverage.rs that asserts the output reuses the parameter VarId and that body references resolve to the shared binding.
• Added runtime regression tests shared_input_output_name_behaves_like_in_out_parameter and shared_input_output_name_preserves_input_value_for_other_outputs in crates/runmat-ignition/tests/functions.rs to verify single-output and multi-output shared-name behaviors at execution time.
• Updated lowering behavior so subsequent remapping and VM frame creation see a single VarId for shared input/output names, restoring MATLAB-compatible semantics.

Testing

• Ran cargo test -p runmat-hir function_output_reuses_param_binding_when_names_match -- --nocapture, which passed.
• Ran cargo test -p runmat-ignition shared_input_output_name -- --nocapture, which passed both added runtime tests.
• Ran cargo fmt --all to format changes.

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Codex Task

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Note

Medium Risk
Changes core function-lowering symbol binding semantics, which can affect variable resolution across many functions; scope is small and covered by new HIR and runtime regression tests.

Overview
Fixes MATLAB in/out parameter semantics during HIR lowering by reusing the existing parameter VarId when a function output name matches an already-defined binding in the current function scope (via new lookup_current_scope).

Adds regression coverage at both the HIR level (verifying shared param/output IDs and body variable resolution) and runtime level in runmat-ignition (verifying single- and multi-output shared-name behavior).

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Project	Deployment	Actions	Updated (UTC)
runmat	Ready	Preview, Comment, Open in v0	Mar 23, 2026 10:39pm