cgen: str N_INDEX read -> 3-word {ptr,len,cap} -- Phase 2 F2 (both stages)

str element value read at N_INDEX dropped the cap word (2-word ptr,len load); str is 24B {ptr,len,cap} since 1140a59. A new named helper cgslicehdr (both stages) loads the full 3-word header and is called by the N_INDEX str-element sites, kind-gated type_isstr/elemisstr -- never size==24, since str and slice collide at 24B. The base-targeting word loads last (clobber-safe). cstage==wwstage byte-identical. The #9 typeassert leaf is split out to F2b (it needs a wwstage spill twin first).

test/wcc/932: table-driven runtime .cap-survives probe over both N_INDEX base forms and both drivers; verified fail-before/pass-after. NNN<950 mirrors the 928 precedent -- the fixtures are self-contained (/tmp, no imports), so rule-14's selfhost-sibling race does not apply.

main.combined.ww regenerated via the canonical make path (md5-stable) and committed alongside source, per the 1140a59 precedent.

Makefile

@@ -254,6 +254,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_match_slice_variant \
 	  $(BIN)/test_match_slice_variant_run \
 	  $(BIN)/test_str_abi_run \
+	  $(BIN)/test_str_elem_cap_run \
 	  $(BIN)/test_composite_call_arg \
 	  $(BIN)/test_composite_call_arg_run \
 	  $(BIN)/test_letdecl_zeroinit \
@@ -620,6 +621,12 @@ $(BIN)/test_str_abi_run: test/wcc/928_str_abi_run.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_str_elem_cap_run: test/wcc/932_str_elem_cap_run.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_composite_call_arg: test/wcc/723_composite_call_arg.c \
 		$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<