feat: execute optional captures in type-producing procedures

TODO.md

@@ -27,8 +27,8 @@ including owned by-value tagged payload captures in ordinary generic procedures.
 type-producing procedures also accept ordinary
 constant inputs, cached and erased like explicit compile-time arguments. mutable
 type/value locals, direct assignments, branches, scalar/enum/tagged-union matching
-and bounded integer-range/while loops now generate types and storage. payload
-captures and matches select nested constructors, lengths, signatures and defaults;
+and bounded integer-range/while loops now generate types and storage. optional
+if/while captures and tagged payload matches select nested constructors, lengths, signatures and defaults;
 general type-valued execution, reflection and generic library acceptance remain open.
 the driver retains frontend storage and source provenance through
 `@source/compilation`, with compilation-wide AST node identities.
@@ -71,12 +71,13 @@ test discovery and FFI remain outside these completed slices.
   - [x] aggregate/explicit-expression checkpoint: computed nested records/tuples and fixed arrays, contextual/dependent aggregate value parameters, normalized specialization caching and explicit `$()` expressions through interpreter/native execution. preserve by-value isolation, zero-cell shapes, integer widths, string ownership, enum/optional fields and exact IEEE float bits; reject runtime dependencies and values that would retain temporary interpreter storage. repeatable compilation restores erased expressions.
   - [x] conditional specialization checkpoint: explicit compile-time `if` guards and side-effect-free constant guards in concrete generic procedures select a body before specialization/checking. type equality retains aliases, nominal identity and structural signatures; composed error-set identity/cache keys ignore ordering, grouping and repetition. guard checking, selected-body diagnostics, scoped cleanup, runtime guards and repeatable compilation work through interpreter/native execution; both arms remain parsed and name-resolved.
   - [x] constant-input type-procedure checkpoint: type-producing procedures accept ordinary parameters alongside `$` parameters, require every argument explicitly and evaluate/cache/erase all inputs. generated record/array storage, nominal cache identity, nested calls, dependent contextual aggregate inputs, exact erased-argument checking, source diagnostics, imported producers and repeatable compilation work through interpreter/native execution.
-  - [x] mutable type-execution checkpoint: type-producing bodies execute mutable type/value locals, direct binding assignments, scoped blocks, if/else and early returns, integer-range/while loops, break/continue and nested/recursive producers. generated nested types, scalar-computed lengths/defaults, nominal cache identity, exact value types, overflow/guard/assignment diagnostics, imported producers and repeatable compilation work through interpreter/native execution. type-control execution shares a 4096-step bound across nested producers; scalar operations reuse checked IR execution. field/index updates, optional captures, cleanup and type-valued aggregate storage in these bodies remain open.
+  - [x] mutable type-execution checkpoint: type-producing bodies execute mutable type/value locals, direct binding assignments, scoped blocks, if/else and early returns, integer-range/while loops, break/continue and nested/recursive producers. generated nested types, scalar-computed lengths/defaults, nominal cache identity, exact value types, overflow/guard/assignment diagnostics, imported producers and repeatable compilation work through interpreter/native execution. type-control execution shares a 4096-step bound across nested producers; scalar operations reuse checked IR execution. field/index updates, cleanup and type-valued aggregate storage in these bodies remain open.
   - [x] type-match checkpoint: type-producing bodies execute scalar/enum match statements and direct type-valued match expressions in initializers, assignments and returns, including nested selected arms. ordinary checking/IR execution select the arm and validate subject/pattern types, ranges, overlap, unreachable arms and exhaustiveness; only the selected body is elaborated. generated storage, scoped locals, loop transfers, early returns, nominal cache identity, selected-body diagnostics, dependency provenance and repeatable compilation work through interpreter/native execution. all bodies remain parsed/name-resolved and execution shares the type-control bound.
   - [x] nested type-match checkpoint: matches select types inside constructors, array lengths, generic arguments, local annotations, callback signatures and field defaults in type-producing bodies. selection precedes ordinary expansion, preserves literal spelling/source provenance and loop snapshots, and caches nominal results before expanding recursive fields. selected-body and pattern diagnostics, failed deferred elaboration, imported producers and repeatable compilation work through interpreter/native execution; deferred selection shares the producer's type-control bound.
   - [x] tagged/fallible constant checkpoint: explicit `$()` expressions, generic value parameters and type-procedure inputs materialize tagged unions and both fallible channels through interpreter/native execution. preserve active tags/payloads, nested raw aggregate projections, string ownership, void/zero-cell variants, composed/payload errors, declared channel types, normalized cache identity and repeatable compilation. `catch` computes generated lengths and runtime values. untagged unions and storage-bearing active payloads remain escape diagnostics rather than retaining temporary interpreter handles.
   - [x] tagged type-match checkpoint: type-producing bodies execute tagged-union matches with immutable by-value payload captures in statement, expression and nested storage positions. ordinary match checking validates every arm's captures, including grouped variants; selected payloads retain exact scalar/aggregate types and owned strings. generated storage/defaults, loop snapshots/transfers, cache identity, selected-body diagnostics, imported producers, dependency provenance and repeatable compilation work through interpreter/native execution. storage-bearing payloads retain the constant-materialization escape boundary.
   - [x] generic match-specialization checkpoint: side-effect-free constant scalar/enum/tagged-union subjects in concrete generic procedures and explicit `match $(subject)` expressions/statements select an arm before body specialization/checking. reuse ordinary checked IR for pattern types, ranges, overlap, exhaustiveness and every arm's capture validation; only the selected body is elaborated. exact by-value payload types/owned strings, runtime parameters, nested expression matches, cleanup, return/loop transfers, imported producers and repeatable compilation work through interpreter/native execution. ordinary call/runtime/mutable subjects stay runtime; contextual aggregates with possible runtime defaults are not implicitly evaluated. all bodies remain parsed/name-resolved; storage-bearing payloads retain the constant-materialization escape boundary.
+  - [x] optional type-control checkpoint: type-producing bodies execute optional if/while guards with immutable by-value payload captures. ordinary checked IR validates guards; selected scalar/aggregate payloads retain exact types and owned strings across nested captures, generated storage/defaults, loop updates/transfers and early returns. absent optionals skip unselected bodies; invalid guards, capture mutation/shadowing, overflow and execution bounds retain diagnostics. imported producers, dependency provenance, nominal cache identity and repeatable compilation work through interpreter/native execution. null data/function pointers materialize correctly; nonnull storage-bearing payloads retain the constant-materialization escape boundary.
   - [ ] remaining gate: general type-valued compile-time execution, inline match/loop expansion and reflection (`typeinfo!`, `field!`, `struct_type!`, generated type/default arrays), broader constant materialization (including untagged unions and storage-bearing values), plus existing-library acceptance. required collection/index iteration, expression-valued control flow, compound updates, formatting parsing and imported C-binding support may interleave with later gates.
 - [ ] remaining source constructs: collection/index iteration, expression-valued control flow and `yield`, compound updates and multiline strings. examples: `source/parser/parse.hon`, `source/llvm/runtime.hon`.
 - [ ] standard-library milestone: compile/run a multi-package program exercising existing allocation and IO, including error cleanup; no replacement miniature standard library.