Add optional narrowing and payload access #28

Goal

Make builtin nullable ?T usable through full-CFG narrowing and ownership-safe payload access.

Settled source semantics

• T promotes directly to ?T; none represents absence.
• value != none proves presence on true edge; value == none proves presence on false edge. Reversed operands behave identically.
• Proven ordinary value use unwraps one optional layer. Nested optionals need one proof per layer.
• Inferred bindings use effective per-use type: let payload = value infers T under proof and ?T outside proof.
• Explicit optional destinations and whole-carrier assignments preserve carrier type.
• Terminating guards preserve surviving facts; joins intersect them; loops run to fixed point.
• && and || remain eager. RHS receives no short-circuit proof, while outgoing edges may use facts implied by completed boolean result.
• Presence checks inspect without consuming.

Stable places

• Narrow variables, fields, nested projections, constant-foldable indexes, and direct resolved integral binding indexes.
• Direct binding indexes carry dependency identity. Assignment to index binding invalidates dependent facts.
• Computed indexes such as items[i + 1] are unstable unless existing const evaluation reduces them.
• Assignment to carrier or ancestor invalidates; payload-descendant mutation preserves outer presence.
• Overlapping alias mutation invalidates affected facts.
• Mutable-reference and known raw-pointer calls invalidate overlapping facts after argument evaluation.
• Unknown raw-pointer effects invalidate all potentially reachable facts. Calls invalidate mutable module-global facts.
• Scope exit removes facts rooted in exited bindings.

Ownership boundary

• Copyable payload and shared-reference payload reads preserve carrier.
• Moving move-only payload from direct named local/parameter optional consumes whole carrier; reinitialization restores it.
• Field/index/pointee payloads may be narrowed and borrowed, but move-only partial extraction is rejected.
• Mutable-reference payload transfer follows existing transfer and borrow-conflict rules.

Compiler work

• Insert explicit FlowTyped phase: Typechecked -> CFG -> FlowTyped -> DefiniteInit -> Ownership -> Usage -> HIR.
• Store per-function site facts, refined expression types, payload evidence, carrier/value origins, and index dependencies in module-owned flow result.
• Reuse canonical typechecker expression logic through refinement injection; do not add a second AST typing switch.
• Reuse and extend internal/semantics/place origin/overlap logic; do not add a second alias walker.
• Add T0041 for unproven payload use and T0042 for unstable narrowing subjects.
• Ownership consumes flow evidence while preserving cleanup planning.
• Add explicit optional-presence and optional-payload HIR/MIR operations.
• LLVM lowers through named present and value layout fields.

Representation and explicit non-scope

• Keep tagged {i1, payload} representation for every optional in this issue.
• Niche layout remains Add optional niche layouts #30 and cannot affect source semantics.
• No Some, unwrap operator, optional pattern, fallback operator, extraction operator, optional chaining, optional-to-optional equality, or new syntax.

Dependencies

• Delivers optional dependency needed by iterable protocol work in Add iterable for loops #31.
• Does not close or absorb Add optional niche layouts #30.

Validation

• Presence polarity, reversed operands, guards, joins, loops, eager boolean results, unreachable paths, and nested optionals.
• Inference, explicit carrier contexts, shadowing, fields, nested fields, constant indexes, binding indexes, and computed-index rejection.
• Exact/ancestor/descendant assignment, alias, call, global, index-dependency, and scope-exit invalidation.
• Copy/borrow/move ownership, direct owner consumption/reinitialization, use-after-consume, conditional cleanup, and rejected partial moves.
• Explicit HIR/MIR operations, tagged 32-bit/64-bit LLVM layout, payload projection, presence checks, and drops.
• Positive runtime plus applicable negative x_test fixtures with bundled compiler.
• LSP refined hover, diagnostics, incremental reset, and snapshot reuse.