comptime compiler-API: Phase 1 foundation + Phase 2.1 weld plan

Introduce the welded comptime `compiler` library (`#library "compiler"` +
`abi(.zig) extern compiler`), per design/comptime-compiler-api.md, and unify
`callconv(...)` into the new `abi(...)` annotation.

abi(...) replaces callconv(...):
- New ABI enum { default, c, zig, pure }; `abi(.c|.zig|.pure)` parses in the
  postfix slot before extern/export (and standalone). `kw_callconv` -> `kw_abi`.
- Migrated 52 sx files, the call-convention-mismatch diagnostic, and docs
  (readme/specs) from `callconv(.c)` to `abi(.c)`.

Phase 1 — welded compiler library (parse -> registry -> validation -> bridge):
- `abi(.zig) extern compiler` parses on fn decls (carries abi/extern_lib) and
  struct decls (StructDecl.abi/extern_lib).
- `#library "compiler"` is the comptime-only internal surface — never dlopen'd.
- src/ir/compiler_lib.zig: the binding registry (the safety boundary). `Field`
  welded to StructInfo.Field with layout baked from the real Zig type
  (@offsetOf/@sizeOf); `findType`/`findFn`. Welded structs are layout-validated
  at registration (field set + total size) as a header checked against the impl.
- Host-call bridge: a `fn abi(.zig) extern compiler` dispatches under the
  comptime interp to its registered Zig handler (intern/text_of round-trip),
  never dlsym. IR Function.compiler_welded; validated in declareFunction.
- Comptime-only enforcement: a runtime call to a welded fn is a clean
  build-gating error (emitCall), not an undefined-symbol link failure.

Phase 2.1 — byte-layout weld foundation:
- Decision: full byte-layout weld (sx struct laid out byte-identically to the
  bound Zig type). Registered StructInfo (first non-natural / Zig-reordered
  layout). `computeWeldPlan` — pure offset-ordered element plan + padding +
  sx-field->LLVM-element remap; unit-tested. Emit/interp wiring is the next
  sub-step (2.2+, see current/CHECKPOINT-COMPILER-API.md).

Examples: 0625/0626 (welded struct + fn round-trip), 1183/1184/1185
(layout-mismatch, unexported-fn, runtime-call diagnostics).

library/modules/ffi/objc.sx

@@ -53,7 +53,7 @@ objc_msgSend          :: (recv: *void, sel: *void)      -> *void  extern objc;
 //
 // IMPs (method implementations) are function pointers with the implicit
 // Obj-C method shape: `(self: *void, _cmd: *void, ...args) -> ret` with
-// `callconv(.c)` so they land args in the standard C registers.
+// `abi(.c)` so they land args in the standard C registers.
 //
 // Method type encoding strings follow Apple's runtime DSL:
 //   v = void   c = char/BOOL   i = int   l = long   f = float   d = double
@@ -120,7 +120,7 @@ impl Into(*NSString) for string {
     convert :: (self: string) -> *NSString {
         cls := objc_getClass("NSString".ptr);
         sel := sel_registerName("stringWithUTF8String:".ptr);
-        msg : (*void, *void, [*]u8) -> *void callconv(.c) = xx objc_msgSend;
+        msg : (*void, *void, [*]u8) -> *void abi(.c) = xx objc_msgSend;
         return xx msg(cls, sel, self.ptr);
     }
 }