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/token.zig

@@ -42,7 +42,7 @@ pub const Tag = enum {
     kw_impl, // impl
     kw_Self, // Self (in protocol declarations)
     kw_inline, // inline (compile-time if/for/while)
-    kw_callconv, // callconv (calling convention annotation)
+    kw_abi, // abi (ABI / calling-convention annotation: abi(.c)/abi(.zig)/abi(.pure))
     kw_extern, // extern (import: external linkage, C ABI, no body)
     kw_export, // export (define + expose: external linkage, C ABI)
     kw_asm, // asm (inline assembly expression / global asm decl)
@@ -281,7 +281,7 @@ pub const keywords = std.StaticStringMap(Tag).initComptime(.{
     .{ "impl", .kw_impl },
     .{ "Self", .kw_Self },
     .{ "inline", .kw_inline },
-    .{ "callconv", .kw_callconv },
+    .{ "abi", .kw_abi },
     .{ "extern", .kw_extern },
     .{ "export", .kw_export },
     // `asm` is a real keyword; `volatile` / `clobbers` stay OUT of this table