Implement struct parameter modifiers (in[], mut[], out[])

[antonsynd]

Context

Identified during spec-test alignment audit (GAP-16). The language specification documents struct parameter modifiers but they are not implemented in the compiler.

Current behavior

Struct parameter modifiers (in[], mut[], out[]) are documented in the specification but are not recognized by the lexer, parser, or semantic analysis. Using them results in a syntax error.

Expected/proposed

Implement the struct parameter modifier syntax as specified:

• in[T] — immutable borrow (pass by readonly reference)
• mut[T] — mutable borrow (pass by reference)
• out[T] — output parameter

These modifiers provide explicit control over struct passing semantics, aligning with the .NET value type model while maintaining Python-like syntax.

Implementation would require changes to:

1. Lexer — recognize in, mut, out as contextual keywords in parameter positions
2. Parser — parse modifier syntax in parameter declarations
3. Semantic — validate modifier usage (e.g., mut only on struct types)
4. CodeGen — emit appropriate ref/in/out parameter modifiers in C#

References

• docs/language_specification/structs.md — struct parameter modifier specification
• docs/language_specification/type_system.md — value type semantics