cgen+test: copy struct >8B local-ident rhs in N_LET (#32)

let p2: T = p1; where T is a struct >8B and rhs is a local ident
silently dropped most of the copy. Cstage's N_LET fell past every
specialized rhs branch (str/tuple/tagged/structlit/call) without
matching the bare-ident case, then past the sz==8 fallback (false)
to the no-rhs zero-init (false: rhs present), emitting zero
instructions — the dest slot read fresh-stack zeros. Wwstage's
cglet fell to cgexpr+MOVQ AX which loads only the first qword
(cgident shape for struct ident), and for sz==16 slots the
str-init tail then stored a stale BX into +8. Reads after the
let saw whatever the stack held: silent partial copy.

Both stages now byte-copy src slot → dst slot per qword with
a sized tail (MOVL/MOVB) for natural sizes not 8-aligned.
Mirrors cg_widen_tagged_store's struct-ident payload copy.

744_letcopy_struct pins the four struct shapes (3×i32, i32+str,
i32+[]u8, i32+tagged) on asm-presence in both stages, cmp -s
byte-id, and runtime exit code via both drivers.

Scope: only N_IDENT rhs at the local-ident-found path. Filed as
siblings (no in-tree consumer today, bootstrap byte-id proves it):
  - N_DOT / N_INDEX / N_UN(deref) struct rhs.
  - Top-level (non-local) struct ident rhs.
  - TY_TUPLE same-shape ident-copy bug.

Row (a) uses tri{a=11, b=22, c=33} structlit init for p1 to
isolate this fix from STATUS-3 #15/#26c (no-rhs zero-init sz=12
vs sz=16 slot-padded divergence between stages, separate task).
Row (d) runtime check uses only p2.a to isolate from match-on-
tagged-field scrutinee spill divergence (same task).

118/118 ok. ww2 == ww3 == ww4 byte-id holds.

Makefile

@@ -276,6 +276,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_dotbase_chained \
 	  $(BIN)/test_parse_error \
 	  $(BIN)/test_variadic_pack \
+	  $(BIN)/test_letcopy_struct \
 	  $(BIN)/test_fnparams_bare_leaf_shadow \
 	  $(BIN)/test_fnret_bare_leaf_shadow \
 	  $(BIN)/test_param_shadow_mod \
@@ -669,6 +670,12 @@ $(BIN)/test_variadic_pack: test/wcc/743_variadic_pack.c \
 		$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_letcopy_struct: test/wcc/744_letcopy_struct.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_fnparams_bare_leaf_shadow: test/wcc/732_fnparams_bare_leaf_shadow.c \
 		$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<