comptime expandable match statements

TODO.md

@@ -851,8 +851,8 @@
   - tuples are unnamed-field structs with structural anonymous values, nominal named declarations,
     brace literals, numeric fields, and no runtime metadata
   - `@std/meta`, `typeinfo!`, `field!`, `compile_error!`, specialization-time branches, and semantic
-    `inline for` use checker-owned persistent compile-time values for aggregate-first reflection and
-    heterogeneous static expansion; inline-loop control is recursively resolved at comptime
+    `expand for` use checker-owned persistent compile-time values for aggregate-first reflection and
+    heterogeneous static expansion; expand-loop control is recursively resolved at comptime
   - interleaved comptime parameters use semantic candidate resolution, immutable byte values specialize
     by contents, and all comptime parameters remain erased from the runtime ABI
   - `io.print(writer, format, args)` validates and expands `{s}` / `{d}` formatting at comptime,
@@ -885,6 +885,16 @@
   - integer output uses one base-aware 65-byte stack buffer; float output uses fixed-buffer
     libc `snprintf` with 32-bit and 64-bit general/scientific precision and propagates failure
 
+40. compile-time `expand` (implemented)
+  - expansion-oriented `inline for` is strictly renamed to `expand for`; `inline` remains available
+    for future function-inlining syntax
+  - final expanded enum and tagged-union match arms generate checker-local specialized arms only
+    for variants not covered by preceding explicit arms
+  - generated enum values and union tags are static bindings, heterogeneous payloads retain their
+    concrete types, and `void` payloads support value and pointer captures without runtime storage
+  - `tag!` reads or folds a tagged union discriminant, while `tagname!` turns a comptime-known enum
+    value into its immutable field-name string
+
 ## A word on unchecked casts
 
 For casts that bypass safety checks, Honey provides builtin functions: