v0.0.12: Match exhaustiveness checking (C6c)

C6c: Match Exhaustiveness Checking

Compile-time verification that match expressions cover all possible values.

Added

• ADT exhaustiveness: all constructors covered or catch-all present
• Bool exhaustiveness: both true and false covered
• Infinite type exhaustiveness: Int, String, Float64, Nat require wildcard
• Unreachable arm warnings
• 17 new type checker tests (570 total)

Closes #18 · PR: #37