v0.0.16: Match expression codegen (C6g)

C6g: Match Expression Codegen

Compile MatchExpr AST nodes to WASM chained if-else cascades.

Added

• ADT tag dispatch: load tag from heap pointer, compare, branch
• Field extraction at computed offsets into locals
• Pattern types: ConstructorPattern, NullaryPattern, WildcardPattern, BindingPattern, BoolPattern, IntPattern
• Recursive if-else cascade with environment scoping per arm
• 20 new codegen tests (643 total)

Closes #26 · PR: #41