Add optional narrowing and payload access

[itsfuad]

Goal

Make builtin nullable ?T usable through control-flow narrowing and safe payload access.

Settled source semantics

• T promotes directly to ?T.
• none represents absence.
• value != none narrows a stable optional binding to T in the true branch.
• value == none narrows that binding in the false branch.
• A terminating guard preserves the surviving branch proof after the guard.
• Assignment to the checked binding invalidates its proof.
• Presence checks inspect without consuming the optional value.

Explicit non-scope

• No Some, some(...), unwrap operator, optional pattern form, fallback operator, extraction operator, or optional chaining syntax.
• No source behavior may depend on tagged versus niche backend layout.
• V1 narrowing applies to stable named bindings. Field and indexed-place narrowing require separate stability evidence.

Compiler work

• Key branch-local narrowing facts by canonical resolved symbol identity.
• Record payload-access evidence during semantic checking; HIR and later phases must consume it without re-resolving source expressions.
• Invalidate facts on assignment and merge facts conservatively across branches and loops.
• Preserve move, borrow, cleanup, and single-owner rules when payload type is move-only.
• Lower presence checks and payload extraction through canonical optional layout APIs for tagged and niche representations.

Dependencies

• Blocks iterable protocol work in Add iterable for loops #31 when iterator next returns ?T.
• Optional niche expansion in Add optional niche layouts #30 is representation-only and must remain source-semantics independent.

Validation

• Typechecker tests for true branch, false branch, reversed comparisons, terminating guards, merge behavior, shadowing, assignment invalidation, and unstable places.
• Ownership tests for copy, borrow, and move-only payloads.
• HIR, MIR, and LLVM tests for explicit presence and payload operations.
• Positive and negative x_test fixtures using bundled compiler.
• Tagged and niche optional coverage on 32-bit and 64-bit targets.