feat: materialize tagged-union and fallible constants

TODO.md

@@ -19,10 +19,11 @@ explicit fallible channels, composed error sets, `try`/`catch` and scoped LIFO
 traps or compiler infrastructure failures.
 cached generic specialization and type-producing procedures support explicit/inferred
 compile-time arguments and generated record/array storage through both paths.
-ordinary procedures can compute scalar and nested record/array compile-time values
-using mutable locals, loops and recursive calls; explicit `$()` expressions use
-the same constant evaluation path. compile-time `if` selection and type comparisons
-specialize only the selected body. type-producing procedures also accept ordinary
+ordinary procedures can compute scalar, nested record/array, tagged-union and
+fallible compile-time values using mutable locals, loops and recursive calls;
+explicit `$()` expressions use the same constant evaluation path. compile-time
+`if` selection and type comparisons 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. matches select nested
@@ -72,7 +73,8 @@ test discovery and FFI remain outside these completed slices.
   - [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. 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.
+  - [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.
+  - [ ] 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 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.
 - [ ] native driver integration: required C ABI declarations, `process.Init` startup arguments/IO, actual CLI exit statuses, file loading, temporary files and spawning Clang. Honey-generated programs currently accept only parameterless `main` and print its result; Bro's bootstrap startup support is not Honey backend support.