compiler-API: welded structs by reflection + memory-order validation

Replace the explored byte-layout-override engine (offset-ordered LLVM structs /
weld plans / byte-blobs — all unnecessary) with a much simpler design: a welded
`struct abi(.zig) extern compiler { … }` is a bodied header declaring its fields
in the bound compiler type's MEMORY order. The compiler reflects the real Zig
type (field names via @typeInfo, offsets via @offsetOf, size via @sizeOf —
nothing hand-maintained) and validates the header matches, with loud diagnostics.

On pass it is an ordinary struct whose natural layout already equals the Zig
layout — no reorder, no padding, no index/remap tables, no special LLVM path — so
@ptrCast'ing it to the compiler's own type and dereferencing is byte-identical.
When types.zig shifts, the header stops matching and the developer gets a specific
message to fix it.

- compiler_lib.zig: weldStruct reflects field names and bakes bound_types fields
  in ascending-offset (memory) order; deleted computeWeldPlan/WeldPlan/WeldElement.
- nominal.zig validateWeldedStruct: precise diagnostics — field-not-found,
  wrong-field-order (+ expected memory order), type-layout (size) mismatch,
  total-size mismatch.
- Examples: 0627 (StructInfo in memory order, byte-identical, usable),
  1186 (source-order StructInfo -> wrong-field-order diagnostic); 1183 refreshed.
- Design doc + checkpoint updated.

examples/1186-diagnostics-weld-struct-wrong-order.sx

@@ -0,0 +1,20 @@
+// Diagnostic: a welded struct whose fields are NOT in the compiler type's memory
+// order is a loud build error — the sx header must mirror the real Zig layout so
+// the two are byte-identical. The message names the offending position and shows
+// the expected memory order. (Declaring StructInfo in source order trips this:
+// Zig reorders it to fields-first.)
+
+#import "modules/std.sx";
+
+compiler :: #library "compiler";
+
+Field :: struct abi(.zig) extern compiler { name: u32; ty: u32; }
+
+StructInfo :: struct abi(.zig) extern compiler {
+    name: u32;
+    fields: []Field;
+    is_protocol: bool;
+    nominal_id: u32;
+}
+
+main :: () { print("unreached\n"); }