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).

src/ir/lower/closure.zig

@@ -78,7 +78,7 @@ pub fn lowerLambda(self: *Lowering, lam: *const ast.Lambda) Ref {
     // Without implicit_ctx, env is slot 0 and user params follow.
     var params = std.ArrayList(Function.Param).empty;
     const env_ptr_ty = self.module.types.ptrTo(.void);
-    const lambda_wants_ctx = self.implicit_ctx_enabled and lam.call_conv != .c;
+    const lambda_wants_ctx = self.implicit_ctx_enabled and lam.abi != .c;
     if (lambda_wants_ctx) {
         params.append(self.alloc, .{
             .name = self.module.types.internString("__sx_ctx"),
@@ -168,7 +168,7 @@ pub fn lowerLambda(self: *Lowering, lam: *const ast.Lambda) Ref {
     };
     const name_id = self.module.types.internString(name);
     const func_id = self.builder.beginFunction(name_id, params.items, ret_ty);
-    if (lam.call_conv == .c) {
+    if (lam.abi == .c) {
         self.module.getFunctionMut(func_id).call_conv = .c;
     }
     self.builder.currentFunc().has_implicit_ctx = lambda_wants_ctx;