Add advanced enum match patterns

[itsfuad]

Extend current enum matches with advanced pattern forms.

Delivered baseline

• PR Add named enums and exhaustive match #98 delivered statement-only exhaustive fully named enum matches.
• Add with variant construction syntax #99 delivered whole-payload binding/discard and struct-field payload destructuring with canonical with syntax.

Remaining scope

• Wildcards.
• Alternatives.
• Guards.
• Nested patterns.
• Shorthand field bindings.
• Exhaustiveness, reachability, binding consistency, ownership, and diagnostics for those forms.

Specify semantics before syntax implementation.