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/sema.zig

@@ -830,7 +830,7 @@ pub const Analyzer = struct {
         switch (node.data) {
             .fn_decl => |fd| {
                 const saved_cc = self.in_c_conv;
-                self.in_c_conv = fd.call_conv == .c;
+                self.in_c_conv = fd.abi == .c;
                 try self.pushScope();
                 try self.analyzeParams(fd.params);
                 try self.analyzeNode(fd.body);
@@ -852,7 +852,7 @@ pub const Analyzer = struct {
                     if (mnode.data == .fn_decl) {
                         const m = mnode.data.fn_decl;
                         const saved_cc = self.in_c_conv;
-                        self.in_c_conv = m.call_conv == .c;
+                        self.in_c_conv = m.abi == .c;
                         try self.pushScope();
                         try self.analyzeParams(m.params);
                         try self.analyzeNode(m.body);
@@ -979,7 +979,7 @@ pub const Analyzer = struct {
             try self.diagnostics.append(self.allocator, .{
                 .level = .warn,
                 .span = span,
-                .message = "`context` is unavailable in a `callconv(.c)` function — the C ABI has no implicit context parameter; pass what you need explicitly",
+                .message = "`context` is unavailable in an `abi(.c)` function — the C ABI has no implicit context parameter; pass what you need explicitly",
             });
             return;
         }
@@ -1032,7 +1032,7 @@ pub const Analyzer = struct {
                     });
                 }
                 const saved_cc = self.in_c_conv;
-                self.in_c_conv = fd.call_conv == .c;
+                self.in_c_conv = fd.abi == .c;
                 try self.pushScope();
                 try self.analyzeParams(fd.params);
                 try self.analyzeNode(fd.body);
@@ -2366,7 +2366,7 @@ test "sema: member references record fields, methods, and enum variants" {
     try std.testing.expect(red_use);
 }
 
-test "sema: context in a callconv(.c) function reports a specific diagnostic" {
+test "sema: context in an abi(.c) function reports a specific diagnostic" {
     const parser_mod = @import("parser.zig");
     var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
     defer arena.deinit();
@@ -2374,7 +2374,7 @@ test "sema: context in a callconv(.c) function reports a specific diagnostic" {
 
     const source =
         "Context :: struct { allocator: i64; data: i64; }" ++
-        "cb :: () -> i64 callconv(.c) { context; 0; }" ++
+        "cb :: () -> i64 abi(.c) { context; 0; }" ++
         "ok :: () -> i64 { context; 0; }";
     var parser = parser_mod.Parser.init(alloc, source);
     const root = try parser.parse();
@@ -2385,7 +2385,7 @@ test "sema: context in a callconv(.c) function reports a specific diagnostic" {
     var c_conv_diag = false;
     var undefined_diag = false;
     for (res.diagnostics) |d| {
-        if (std.mem.indexOf(u8, d.message, "callconv(.c)") != null) c_conv_diag = true;
+        if (std.mem.indexOf(u8, d.message, "abi(.c)") != null) c_conv_diag = true;
         if (std.mem.indexOf(u8, d.message, "undefined") != null) undefined_diag = true;
     }
     try std.testing.expect(c_conv_diag); // `cb` accesses context under the C ABI