comptime-API: strip the byte-weld; pivot to a flat-memory comptime VM

The byte-weld (sx structs whose layout was validated to mirror the
compiler's Zig records) plus the serialization/marshaling bridge was the
wrong direction: it bolted a parallel layout regime and hand-built
byte-copies onto a comptime value model that fundamentally isn't bytes.

Strip the struct-weld machinery:
- compiler_lib.zig loses the type registry (weldStruct / bound_types /
  BoundType / FieldLayout / findType / SxField / LayoutMismatch /
  validateStructLayout); it is now just the intern/text_of function
  host-call bridge (kept as the Phase-3 compiler-call seed).
- nominal.zig loses validateWeldedStruct / weldedFieldOrderStr + the
  sd.abi == .zig validation call.
- Remove the struct-weld unit tests and examples 0625/0627 (welded
  structs) + 1183/1186 (weld-layout diagnostics).
- The #library / abi / extern syntax stays.

Record the new direction: a bytecode VM over flat, byte-addressable
memory so comptime values are native bytes (no weld/validation/marshal),
target-aware (preserves cross-compilation) and sandboxed. See
current/PLAN-COMPILER-VM.md (Phase 0 strip -> Phase 1 flat-memory value
model -> Phase 2 bytecode -> Phase 3 compiler-API on flat memory).
design/comptime-compiler-api.md gets a SUPERSEDED banner. Also drop the
"~500 lines / split the step" rule from CLAUDE.md.

examples/expected/0625-comptime-weld-struct-field.exit

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

examples/expected/0625-comptime-weld-struct-field.stderr

@@ -1 +0,0 @@
-

examples/expected/0625-comptime-weld-struct-field.stdout

@@ -1 +0,0 @@
-name=7 ty=3

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

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

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

@@ -1 +0,0 @@
-

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

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

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

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

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

@@ -1,5 +0,0 @@
-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/1183-diagnostics-weld-struct-field-count.stdout

@@ -1 +0,0 @@
-

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

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

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

@@ -1,5 +0,0 @@
-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

@@ -1 +0,0 @@
-