c interop type foundation

TODO.md

@@ -4,8 +4,10 @@
 
 # milestones
 
-1. interop type foundation
+1. interop type foundation (implemented)
    - unsigned integers, floats, and target-dependent c scalar types
+   - atomic `c_*` primitive types remain distinct until target-aware lowering
+   - `c_func` and pointer-only `c_struct`; `c` remains an ordinary identifier
    - keep binding mutability (`::` / `=`) separate from element or pointee mutability (`mut`)
    - arrays and indexing
      - `[N]T`: array with `N` logical elements
@@ -13,7 +15,7 @@
    - pointers
      - `@T` / `@mut T`: non-null single-item pointer without arithmetic
      - `*T` / `*mut T`: non-null many-item pointer with arithmetic
-     - optional pointers represent nullable pointers
+     - optional pointers represent nullable pointers (i.e. `?@T` / `?@mut T`, `?*T` / `?*mut T`)
    - slices and slicing
      - `[]T`: pointer and length
      - `[;S]T`: pointer and length with a sentinel invariant
@@ -22,9 +24,9 @@
    - character literals
    - optionals with trapping unwrap and fallback operations
    - native structs with compiler-controlled layout
-   - pointer-only `c struct` support with target c layout
-     - `Some :: c struct { ... }`: defined c-layout struct
-     - `Some :: c struct`: opaque c-layout struct
+   - pointer-only `c_struct` support with target c layout
+     - `Some :: c_struct { ... }`: defined c-layout struct
+     - `Some :: c_struct`: opaque c-layout struct
      - defer passing c structs by value until target ABI classification exists
 
 2. restricted c header imports