Closes #22

Summary

Allow payload-bearing constructor patterns to omit unused binders in pattern matching. Users can now write Ok -> ... and Err -> ... instead of requiring Ok(_) -> ... and Err(_) -> ... when the payload is intentionally ignored.

Changes:

• Type checker (mesh-typeck/src/infer.rs): Treat bare payload-bearing constructors as having wildcard arguments for all fields; recognize uppercase identifiers with function types as constructors and return their result type
• Code generator (mesh-codegen/src/mir/lower.rs): Generate MirPattern::Constructor with wildcard patterns for all variant fields when no explicit binder is provided
• Tests: Added exhaustiveness and e2e tests covering Ok/Err, Some/None, and mixed bare/explicit forms

This eliminates repetitive placeholder noise in Result/Option-heavy code while maintaining exhaustiveness checking and type safety.

Verification

# Run new exhaustiveness tests
cargo test --package mesh-typeck --test exhaustiveness_integration -- test_bare
# Result: 3/3 tests passed (bare Ok/Err, bare Some, mixed forms)

# Run new e2e tests
cargo test --package meshc --test e2e -- e2e_bare_payload_constructor
# Result: 2/2 tests passed (Err arm, Ok arm execution)

# Verify no regressions in exhaustiveness checking
cargo test --package mesh-typeck --test exhaustiveness_integration
# Result: 19/19 tests passed (all existing + new tests)