Add optional inspect and unwrap operations

[itsfuad]

Goal

Finish user-facing operations for ?T.

Scope

• Choose syntax for unwrap/check: unwrap operator, is none / is some, or pattern form.
• Add control-flow narrowing only after syntax is settled.
• Keep source semantics independent from niche/tagged backend layout.

Validation

• Typechecker tests for unwrap/check.
• HIR/MIR/backend tests for branching and unwrap.
• Smoke tests for both none and some paths.