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_block.sx

@@ -63,7 +63,7 @@ __sx_block_descriptor : BlockDescriptor = .{
 // `$args` is the bound pack of arg types and `$R` is the bound
 // return type. The `#insert` evaluates `build_block_convert` at
 // comptime and substitutes the resulting source — a nested
-// `callconv(.c)` trampoline + the Block literal that points its
+// `abi(.c)` trampoline + the Block literal that points its
 // `invoke` slot at it. One impl in stdlib replaces every per-
 // signature hand-rolled `__block_invoke_*` + `Into(Block)` pair.
 impl Into(Block) for Closure(..$args) -> $R {
@@ -77,7 +77,7 @@ impl Into(Block) for Closure(..$args) -> $R {
 // (`args`, a comptime `[]Type`) and the closure's return type
 // (`$ret`), emits source that:
 //
-//   1. Declares a nested `__invoke` `callconv(.c)` trampoline whose
+//   1. Declares a nested `__invoke` `abi(.c)` trampoline whose
 //      signature matches the per-shape Apple Block ABI: first arg is
 //      `block_self: *Block`, then the pack types verbatim. The body
 //      reconstructs a typed fn-pointer from `block_self.sx_fn`,
@@ -105,7 +105,7 @@ build_block_convert :: (args: []Type, $ret: Type) -> string {
     }
     code = concat(code, ") -> ");
     code = concat(code, ret_name);
-    code = concat(code, " callconv(.c) { typed_fn : (*void");
+    code = concat(code, " abi(.c) { typed_fn : (*void");
     i = 0;
     while i < args.len {
         code = concat(code, ", ");