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/expr.zig

@@ -1894,7 +1894,7 @@ pub fn lowerExpr(self: *Lowering, node: *const Node) Ref {
                             // Coercing a bare fn name to a fn-pointer
                             // type — the call_conv must match. A
                             // default-conv sx fn assigned to a
-                            // callconv(.c) slot (e.g. passed to
+                            // abi(.c) slot (e.g. passed to
                             // pthread_create) would otherwise crash at
                             // runtime when the C caller doesn't supply
                             // the implicit __sx_ctx arg.
@@ -1902,8 +1902,8 @@ pub fn lowerExpr(self: *Lowering, node: *const Node) Ref {
                                 const func_cc = self.module.functions.items[@intFromEnum(fid)].call_conv;
                                 if (func_cc != tt_info.function.call_conv) {
                                     if (self.diagnostics) |d| {
-                                        const want_cc = if (tt_info.function.call_conv == .c) "callconv(.c)" else "default sx convention";
-                                        const have_cc = if (func_cc == .c) "callconv(.c)" else "default sx convention";
+                                        const want_cc = if (tt_info.function.call_conv == .c) "abi(.c)" else "default sx convention";
+                                        const have_cc = if (func_cc == .c) "abi(.c)" else "default sx convention";
                                         d.addFmt(.err, node.span, "call-convention mismatch: '{s}' is declared with {s} but the target type expects {s}", .{ eff_fn_name, have_cc, want_cc });
                                     }
                                     break :blk self.emitPlaceholder(eff_fn_name);