wcc: struct-composite let/def DATA emit via SSoT helper (#129 A.2)

Extract emit_struct_data + emit_struct_lit_bytes helpers (both stages,
mirrored) for module-level let/def with N_STRUCTLIT initializer. Walks
Tfield linked-list in declaration order, zero-fills padding via per-
field offset (rule 13, no hardcoded sizes), dispatches per field kind:
integer via fold_int_literal, float via inline bitcast + sign-XOR byte-
loop (A.1 shape, no INT64_MIN materialised — sibling #144), nested
struct via recursion (#145 inner-field-name-leak gates the test row).
Out-of-scope field kinds (str/slice/ptr/array) fatal loud per rule 7.

LOAD-side widened symmetric to A.1 precedent: cstage cgexpr N_DOT
direct-struct-ident + chained-N_DOT widened via new DefStruct registry
(def_isstructdef populated in let_collect); wwstage cgdot direct-struct-
global falls through to defvarstructinfo on letvarstructinfo nil
(defent.dtnode field added, populated in collectdefs). Both stages
materialise struct-def via LEAQ name(SB) same as struct-let.

Pre-existing cstage scalar 8B short-circuit at emit_lets caused silent
fold-fail-continue on 8B struct lits (`struct{i32,i32}`); gate now
excludes let_isstruct so 8B struct lits route through emit_struct_data.
Wwstage's `!issg` gate was already correct; symmetric ordering restored.

Closes (all bootstrap-NEUTRAL pre-impl; γ-cleanup #40 first consumer):
- emit_lets `is_struct continue` skip → struct lets emitted no DATA
- emit_defs no struct arm → struct defs emitted no DATA
- cstage cgexpr N_DOT for struct-def emitted MOVSXD (BP), AX (broken
  stack-frame read)
- cstage emit_lets sz==8 short-circuit silently skipped 8B struct lits

Test 918 (7 rows: let_int_struct / def_int_struct / let_float_field /
def_float_field / let_empty_struct / let_int_struct_8b / let_norhs_
struct_regression) registered. Make test: 181/181 incl. 990-997 byte-id
+ combined_ww_fresh.

Followups filed:
- #145 (task #41) — nested struct-lit inner field-name leaks as extern
- #42 — wwstage dotchainresolve missing defvarstructinfo lookup (A.2-
  scope-clean today; surfaces post-#145 nested-struct shapes)
- A.3 (task #39) — array static-init audit (parks shape-4 array-in-struct)
- γ-cleanup (task #40) — lib/math const-floatinfo re-fold, blocked-by A.2

selfhost/cmd/wcc/cgenexpr.ww

@@ -1964,9 +1964,15 @@ fn cgdot(c: *cgen, n: *node) void = {
 	// load at fi.foff(CX). Mirrors the local "Direct struct local"
 	// branch above, swapping the BP frame slot for the global VA.
 	// Field-width-aware op handles MOVQ / MOVL / MOVZBQ / MOVSXD.
+	// #129 A.2: also handles struct-typed `def`s via defvarstructinfo;
+	// emitstructdata gives them DATA storage at name(SB), and this
+	// LEAQ-and-offset shape mirrors the let path. Pre-A.2 the def
+	// fell through to the integer-let MOVQ catch-all (reading garbage
+	// from the wrong offset).
 	if (lhs != nil) {
 		if (lhs.kind == nkind.N_IDENT) {
 			let si: *structinfo = letvarstructinfo(c, lhs.str);
+			if (si == nil) { si = defvarstructinfo(c, lhs.str); };
 			if (si != nil) {
 				let fi: *fieldinfo = si.fields;
 				for (fi != nil) {