Extend C codegen for multi-module output

[heartwilltell]

Phase 5: Compiler Integration — RFC #218

Update src/codegen_c.zig to produce correct C code when compiling multiple modules together. Include proper header declarations for cross-module references and ensure symbol visibility is correct.

Acceptance Criteria

• Generate forward declarations for functions from external packages
• Generate struct definitions from external packages when referenced
• Correct #include ordering for cross-module dependencies
• Avoid duplicate definitions when the same type is used across modules
• Generated C code compiles without warnings with zig cc

Files

• Modified: src/codegen_c.zig

Dependencies

• Implement multi-file compilation for external packages #314 (multi-file compilation)
• Extend IR lowering for cross-package function calls #315 (cross-package IR)

Part of the Package Manager RFC.