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/expected/0627-comptime-weld-struct-reflected-layout.exit

@@ -0,0 +1 @@
+0

examples/expected/0627-comptime-weld-struct-reflected-layout.stderr

@@ -0,0 +1 @@
+

examples/expected/0627-comptime-weld-struct-reflected-layout.stdout

@@ -0,0 +1 @@
+name=42 nominal=7 proto=true

examples/expected/1183-diagnostics-weld-struct-field-count.stderr

@@ -1,4 +1,4 @@
-error: welded type 'Field' has 2 field(s) in the compiler library but the declaration has 1
+error: welded type 'Field': the compiler type has 2 field(s) but the declaration has 1 — declare them in memory order: name, ty
   --> examples/1183-diagnostics-weld-struct-field-count.sx:12:51
    |
 12 | Field :: struct abi(.zig) extern compiler { name: u32; }

examples/expected/1186-diagnostics-weld-struct-wrong-order.exit

@@ -0,0 +1 @@
+1

examples/expected/1186-diagnostics-weld-struct-wrong-order.stderr

@@ -0,0 +1,5 @@
+error: welded type 'StructInfo': wrong field order at position 0 — found 'name', the compiler type has 'fields' here (memory order: fields, name, nominal_id, is_protocol)
+  --> examples/1186-diagnostics-weld-struct-wrong-order.sx:14:11
+   |
+14 |     name: u32;
+   |           ^^^

examples/expected/1186-diagnostics-weld-struct-wrong-order.stdout

@@ -0,0 +1 @@
+