feat: support optional-capture conditional expressions

Reuse canonical matches for optional presence and exact by-value payload captures across checking, lowering and compile-time selection.

Snapshot the optional-match flag before selector construction: AST additions can reallocate storage and invalidate the borrowed tree until refresh.

Verified 268 bootstrap tests and 908 interpreter/native programs at -O0 and -O2, plus native allocator, LLVM and CLI checks.

README.md

@@ -200,7 +200,7 @@ enums use `Kind :: enum { number, empty }`, qualified members such as `Kind.numb
 
 untagged `Bits :: union { integer u64, real f64 }` overlays its members in shared storage. keyed construction selects exactly one member; reading another member reinterprets the same bytes. tagged unions use `union(enum)` or `union(Kind)`; an explicit enum must contain every union variant. construct them with `Choice { number = 42 }`, contextual `.number{42}`, `.point{x = 40, y = 2}` for record payloads, or `.empty` for `void` payloads. field reads/writes validate the active tag. nested aggregates copy by value, including across calls. CLI union results print `{payload}` (or `{}` for a void payload); untagged results print the first member.
 
-`if (guard) then_value else else_value` is a boolean value expression. the guard runs once and only the selected expression executes; `else` is required and binds to the nearest conditional. both runtime branches must fit the result context, including optional/fallible channels and aggregate copies. constant-input procedures, direct `$()` evaluation, generic branch specialization and generated type positions reuse checked match execution. guards require parentheses unless they are calls or explicit `$()` expressions. parenthesize a conditional used as a match-arm value to distinguish it from an `if` statement. optional payload captures and yielding value blocks remain unsupported.
+`if (guard) then_value else else_value` is a boolean value expression. `if value |payload| then_value else else_value` instead tests optional presence and binds the exact payload as an immutable by-value capture in the then expression; nested optionals preserve present-null payloads. the guard runs once and only the selected expression executes; `else` is required and binds to the nearest conditional. both runtime branches must fit the result context, including optional/fallible channels and aggregate copies. constant-input procedures, direct `$()` evaluation, generic branch specialization, explicit compile-time subject selection and generated type positions reuse checked match execution. boolean guards require parentheses unless they are calls or explicit `$()` expressions; optional captures also permit bare guards. parenthesize a conditional used as a match-arm value to distinguish it from an `if` statement. yielding value blocks remain unsupported.
 
 `match value { .number |n|: n + 2, .empty: 0 }` selects one arm after evaluating its subject once. matches support enums, tagged unions, composed error sets, numeric/boolean scalars, comma-separated patterns, numeric ranges, final `else` arms, arm-local payload captures, statement bodies and expression results. checking rejects missing coverage, duplicate/overlapping patterns, unreachable arms and incompatible captures/results. block arms can return or transfer loop control; yielding value blocks and pointer captures belong to later gates.