docs(metatype): struct widening done; tuple is the last shape

current/CHECKPOINT-METATYPE.md

@@ -4,17 +4,26 @@ Companion to [PLAN-METATYPE.md](PLAN-METATYPE.md). Update after every step (one
 step at a time, per the cadence rule).
 
 ## Last completed step
+**`type_info` / `define` widened to STRUCT types.** `TypeInfo` gained a
+`` `struct(StructInfo) `` variant (`StructField{ name, type }`); the metatype
+system now reflects AND constructs structs, not only enums.
+- `meta.sx`: `StructField` / `StructInfo` / `` `struct `` TypeInfo variant.
+- `interp.zig`: `reflectTypeInfo` builds `.struct` (tag 1) for a source
+  `@"struct"`; `define` dispatches on the TypeInfo tag (`defineType` →
+  `defineEnum` (0) / `defineStruct` (1)). `defineStruct` mirrors `defineEnum`
+  (duplicate-field-name check included) but completes the declare slot AS a
+  struct via `replaceKeyedInfo` — a KIND change re-keys the intern map, whereas
+  `updatePreservingKey` (the enum path) asserts the key is unchanged.
+- `lower/call.zig`: the lower-time `type_info` guard now admits `@"struct"`.
+- `examples/0622`: programmatic `Vec2` via `.struct(.{ fields = … })` + a
+  source-struct round-trip `define(declare("RowCopy"), type_info(Row))`. Enum
+  path (`0619`) unchanged. Suite green (683). Tuple is the last shape (Next step).
+
+## Earlier — make_enum
 **`make_enum(name, variants: []EnumVariant) -> Type`** — the general enum
 constructor over `declare`/`define`, minting a nominal enum from a variant list
-passed as a VALUE (not a hardcoded literal). Pure sx in `meta.sx`, no compiler
+passed as a VALUE. Pure sx in `meta.sx`. `examples/0620` assembles the list in a
-machinery — the open-ended form the channel-result constructors are special cases
+local then mints, exercising `define`'s value-arg SLICE decode.
-of. Because `variants` is an ordinary comptime value, a non-generic builder
-assembles it in a local before minting; `examples/0620` (`build_level` fills a
-local array → `make_enum` mints `Level`) exercises `define` decoding a value-arg
-SLICE (`decodeVariantElements`' slice branch) vs. the inline `.[ … ]` array the
-0614–0618 examples pass directly. No compiler change (locks existing capability).
-Suite green (678). See "make_enum follow-ups" under Next step for the deferred
-free-form-construction gaps (subslice/List at comptime, generic-type-fn locals).
 
 ## Prior step
 **`type_info($T)` reflection — enum round-trip.** Reflect a type INTO a `TypeInfo`
@@ -78,9 +87,10 @@ traversal)**; `field_type` reflection `0616`. Full suite green (674 examples).
   (reflection). No constructor-name knowledge anywhere in the compiler — every
   named constructor is sx. `declare(name)` carries the type name (compile-time
   string) for forward-type registration.
-- `type_info($T)` reflects an `enum`/`tagged_union` INTO a `TypeInfo` value
+- `type_info($T)` reflects an `enum`/`tagged_union`/`struct` INTO a `TypeInfo`
-  (`call.zig` emits `callBuiltin(.type_info)`; `interp.zig:reflectTypeInfo` builds
+  value (`call.zig` emits `callBuiltin(.type_info)`; `interp.zig:reflectTypeInfo`
-  the Value). Enum-only; struct/tuple widening pending. `examples/0619` round-trip.
+  builds the Value). `define` decodes `.enum` → tagged_union and `.struct` →
+  struct. Tuple widening pending. `examples/0619` (enum) / `0622` (struct) round-trip.
 
 ## Decision (kept)
 **Meta lives in `modules/std/meta.sx`, not the prelude.** Declaring its data types
@@ -89,10 +99,10 @@ shifts every `.ir` snapshot. On-demand import keeps the prelude clean.
 
 ## Next step
 Pick any (independent):
-- **Widen `type_info` / `TypeInfo` past `` `enum ``** — struct/tuple variants:
+- **Widen `type_info` / `TypeInfo` to `` `tuple ``** — the last shape. `` `enum ``
-  add `` .`struct ``/`` .`tuple `` to the `TypeInfo` enum in `meta.sx`, teach
+  and `` `struct `` now ship (`examples/0619`/`0622`). A tuple has positional
-  `reflectTypeInfo` to build them, and teach `defineEnum` (→ a `defineType`) to
+  (unnamed) element types; needs a `TupleInfo { elements: []Type }` in `meta.sx`,
-  decode them. Round-trips a struct through `define` once it lands.
+  a `reflectTypeInfo` arm for `.tuple`, and a `defineTuple` (mints a `.tuple` type).
 - ~~**Validation + loud diagnostics**~~ — COMPLETE. duplicate variant names
   (`examples/1180`); `declare()` never `define()`d (`examples/1181`, was a
   `verifySizes` panic); by-value self-reference for both source (`1178`) and
@@ -135,6 +145,12 @@ capabilities would let the variant list be built more freely; both error cleanly
   `examples/1178` locks it).
 
 ## Log
+- **Struct widening done.** `TypeInfo` gained `` `struct(StructInfo) ``; `define`
+  dispatches on the tag (`defineType` → `defineEnum`/`defineStruct`), `reflectTypeInfo`
+  reflects a `@"struct"`, and the lower-time `type_info` guard admits structs.
+  `defineStruct` uses `replaceKeyedInfo` (kind change: tagged_union declare slot →
+  struct). `examples/0622` (programmatic build + source round-trip). Suite green
+  (683). Tuple is the last remaining shape.
 - **Validation story COMPLETE.** use-before-define needs no new check — subsumed
   by `checkInfiniteSize` (by-value cycles), declare-never-defined (unfinished
   slots), 0140 bails (bad payloads), and in-place slot mutation (forward refs);