feat: execute nested compile-time type matches

TODO.md

@@ -25,8 +25,9 @@ the same constant evaluation path. compile-time `if` selection and type comparis
 specialize only the selected body. 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 matching and bounded
-integer-range/while loops now generate types and storage; general type-valued
-execution, reflection and generic library acceptance remain open.
+integer-range/while loops now generate types and storage. 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.
 file commands retain single-file behavior; directory commands compile the
@@ -69,7 +70,8 @@ test discovery and FFI remain outside these completed slices.
   - [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] 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. matches nested inside type constructors/defaults and tagged-union payload captures 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. tagged-union payload captures remain open.
+  - [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.
   - [ ] remaining gate: general type-valued compile-time execution, broader compile-time match/inline expansion and reflection (`typeinfo!`, `field!`, `struct_type!`, generated type/default arrays), broader constant materialization (including unions, fallible results 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.