design: ground compiler-API build order in code anchors

#library already lexes/parses (library_decl node); extern/export are
keywords. Phase 1 new work pinned to concrete sites: parser (extern(.zig)
postfix at the #builtin/#compiler positions), AST binding field,
compiler_hooks.zig as the registry, types/llvm layout emission, host_ffi
comptime bridge. First testable sub-step: extern(.zig) <lib> parses on a
fn decl.

design/comptime-compiler-api.md

@@ -148,12 +148,38 @@ special paths: lexer/parser token + sema handling (`src/lexer.zig`, `src/parser.
 registration in `compiler_hooks.zig` (the registry stays, re-homed under `compiler`).
 sx footprint is tiny (2 lines in `library/modules/build.sx`).
 
+## Code anchors (confirmed 2026-06-17)
+
+Foundation that ALREADY exists:
+- `#library "name"` lexes (`hash_library`, `src/lexer.zig:91`) and parses into a
+  `library_decl { lib_name, name }` AST node (`src/parser.zig:210`). So
+  `compiler :: #library "compiler";` works today (used for FFI libs like raylib).
+- `extern` / `export` are keywords (`src/token.zig:46`, `kw_extern`/`kw_export`).
+
+New work for Phase 1:
+- **Lexer/parser**: the `(.zig)` ABI qualifier on `extern`, and the trailing
+  `<lib>` identifier — `… extern(.zig) <lib>` postfix on FN decls (after the
+  return type, beside `#builtin`/`#compiler` at `src/parser.zig:233`/`:315`) and
+  STRUCT decls (beside `struct #compiler`, `src/parser.zig:953`).
+- **AST**: an abi/layout-binding field on `FnDecl` and the struct decl (`abi:
+  .c | .zig`, `lib: ?name`).
+- **Binding registry**: re-home / generalize `src/ir/compiler_hooks.zig` (today's
+  `#compiler` registry) into the `compiler` lib's type+function registry, keyed by
+  exported sx name → Zig type (`@offsetOf` layout) / Zig fn (host-call).
+- **Layout + emit**: sx struct layout (`src/ir/types.zig` / lowering) honors the
+  bound type's offsets; LLVM emission (`src/backend/llvm/types.zig`) hits them.
+- **Host-call bridge**: extend the comptime path (`src/ir/host_ffi.zig` +
+  `interp.zig`) to dispatch `compiler` functions to their registered Zig fns,
+  comptime-only.
+
 ## Build order (each phase keeps `zig build test` green)
 
-1. **`extern(.zig)` + `#library` foundation** — lex/parse the postfix attribute and
-   `#library "compiler"`; a binding registry (sx name → Zig type/fn); the layout
-   engine honoring the bound type's `@offsetOf` offsets + LLVM emission that hits
-   them; **build-time layout-equality assertion**. Prove with `Field` (two u32s).
+1. **`extern(.zig)` + `#library` foundation** — parse the postfix attribute (the
+   `#library` decl already exists); a binding registry (sx name → Zig type/fn);
+   the layout engine honoring the bound type's `@offsetOf` offsets + LLVM emission
+   that hits them; **build-time layout-equality assertion**. Prove with `Field`
+   (two u32s). First testable sub-step: `extern(.zig) <lib>` PARSES on a fn decl
+   (parser unit test), AST carries the binding — no semantics yet.
 2. **Weld `StructInfo`** + `StringId` accessors (`intern`/`text_of`) over the
    host-call bridge.
 3. **Re-express `type_info`/`define` (struct)** as sx over `register_struct`/