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).

examples/0206-generics-generic-into-block.sx

@@ -1,7 +1,7 @@
 // FFI plan step 5.2 — generic `Into(Block) for Closure(..$args) ->
 // $R` impl. One impl in stdlib covers every closure shape; the
 // compiler monomorphises the impl body per call shape and emits a
-// dedicated `__invoke` `callconv(.c)` trampoline + Block literal
+// dedicated `__invoke` `abi(.c)` trampoline + Block literal
 // (via `#insert build_block_convert($args, $R);`).
 //
 // This test exercises a closure shape (`Closure(i64, i64) -> void`)
@@ -13,7 +13,7 @@
 // the per-shape trampoline ferries control back to the sx closure.
 //
 // The block is invoked directly through `b.invoke` (a typed
-// `callconv(.c)` fn-pointer) — the same shape the Apple Block
+// `abi(.c)` fn-pointer) — the same shape the Apple Block
 // runtime calls when a UIKit/Foundation API hands the block back
 // to its registered invoke.
 
@@ -27,7 +27,7 @@ main :: () -> i32 {
     cl := (a: i64, b: i64) => { g_a = a; g_b = b; };
     blk : Block = xx cl;
 
-    invoke_fn : (*Block, i64, i64) -> void callconv(.c) = xx blk.invoke;
+    invoke_fn : (*Block, i64, i64) -> void abi(.c) = xx blk.invoke;
     invoke_fn(@blk, 10, 20);
 
     if g_a != 10 { print("FAIL: g_a={}\n", g_a); return 1; }