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/backend/llvm/abi.zig

@@ -29,12 +29,12 @@ pub const AbiLowering = struct {
     /// `is_extern_c_api` knob. When true, sx `string` / `[]T` slices
     /// collapse to `ptr` — the libc convention where the user writes
     /// `string` to mean `char *` and the length is dropped. When
-    /// false (sx-internal `callconv(.c)` like block trampolines), the
+    /// false (sx-internal `abi(.c)` like block trampolines), the
     /// full slice shape is preserved and goes through the general
     /// struct-coerce path (16-byte slice → `[2 x i64]`, lands in two
     /// registers on AArch64 — the true C ABI for a 16-byte
     /// aggregate). Without the split, sx-to-sx calls through a
-    /// `(*Block, string) -> void callconv(.c)` fn-pointer mismatched
+    /// `(*Block, string) -> void abi(.c)` fn-pointer mismatched
     /// the caller's `{ptr, i64}` value against the trampoline's
     /// collapsed `ptr` param.
     pub fn abiCoerceParamTypeEx(self: AbiLowering, ir_ty: TypeId, llvm_ty: c.LLVMTypeRef, is_extern_c_api: bool) c.LLVMTypeRef {