feat: materialize computed compile-time callbacks

TODO.md

@@ -19,8 +19,8 @@ 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, nested record/array, tagged-union and
-fallible compile-time values using mutable locals, loops and recursive calls;
+ordinary procedures can compute scalar, nested record/array, tagged-union,
+fallible and callback compile-time values using mutable locals, loops and recursive calls;
 explicit `$()` expressions use the same constant evaluation path. compile-time
 `if`/`match` selection and type comparisons specialize only the selected body,
 including owned by-value tagged payload captures in ordinary generic procedures.
@@ -142,6 +142,7 @@ test discovery and FFI remain outside these completed slices.
   - [x] compound/bitwise checkpoint: arithmetic and bitwise compound updates evaluate mutable local/field/index/pointer places and their old values once before the RHS through interpreter/native execution. integer complement, and/or/xor, truncating left shifts, signed/unsigned right shifts and saturating left shifts preserve widths and distinct identity; unsigned counts retain independent types. known invalid counts diagnose even in untaken runtime branches; dynamic counts trap without native LLVM poison. checked arithmetic, float-only division, readonly diagnostics, scoped cleanup, ordinary constants, generated storage, generic branch selection and repeatable compilation retain existing semantics. type-valued `|` remains composition while runtime `|` and capture delimiters retain their grammar. full hash-map acceptance remains open.
   - [x] hash-map execution checkpoint: existing `std/hashmap.HashMap`, `AutoHashMap` and `StringHashMap` run collision chains, zero-hash normalization, growth/rehashing, duplicate-key errors, absent lookups and owned aggregate values without replacement library code. interpreter checks use separate typed allocation buffers and preserve the original map on allocation failure, including cleanup; native `mem.c_allocator` checks cover all integer widths/boundaries, booleans, sparse/aliased enums, nested distinct/array/record keys with padding ignored, content-based byte strings and custom callbacks at `-O0`/`-O2`. specialization retains `usize` collection indices, nominal/explicit pointer-cast result types, rigid slice borrowing/string context and source-known optional callbacks; ordinary checking rejects incompatible elements and readonly reversal. `unreachable` demand-traps with provenance through both paths and compile-time execution; literal-true loops distinguish escaping and nested breaks. `compile_error!` owns computed string diagnostics and skips unselected compile-time bodies. initializer inference from expected result types is covered below.
   - [x] expected-result inference checkpoint: omitted compile-time type/value arguments infer from rigid runtime arguments and expected results before contextual literal defaults. typed bindings, assignments, returns, ordinary/nested calls, aggregate fields/elements/defaults, optional/fallible channels, callbacks, match arms, yielding/catch blocks, arithmetic/bitwise operands and explicit `$()` expressions retain exact checked types through interpreter/native execution. generated nominal results infer counts and private/custom callback identities; existing `hashmap.init(allocator)` initializes `AutoHashMap`, `StringHashMap` and custom `HashMap` without replacement library code. rigid conflicts, unrelated nominal generators and unresolved arguments diagnose; explicit calls and fully explicit type-producer inputs retain their contracts. owned specializations and repeat compilation are verified.
+  - [x] computed-callback materialization checkpoint: ordinary compile-time calls return source-bound callbacks through interpreter/native execution, including optional/nested presence, arrays/records, tagged payloads and both fallible channels. temporary IR function IDs map back to exact source declarations before evaluation storage is released. computed/direct aggregate arguments share specialization-cache identity and generated defaults; imported private targets, same-spelled functions, signature diagnostics, runtime trap provenance and repeated compilation retain their contracts. nonnull data pointers, slices and untagged unions retain their escape boundary.
   - [ ] remaining gate: general type-valued compile-time execution, broader constant materialization (including untagged unions and storage-bearing values), plus remaining existing-library acceptance for broader `meta` operations and `io.print`. formatting parsing and broader imported C-binding support may interleave with later gates.
 - [ ] remaining source constructs: multiline strings. boolean and optional-capture conditional expressions, yielding value blocks, `yield`, compound updates and bitwise operators are covered above. 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.