w6c+wwstage: aggregate return from any addressable source (#272 commit-1)

The N_RETURN aggregate arms gated the return source on N_IDENT ||
N_STRUCTLIT; every other aggregate rvalue (array literal, o.field N_DOT,
a[i] N_INDEX, *p deref) fell through to the scalar-AX default = a silent
8-byte truncation. Both stages emitted byte-IDENTICAL wrong asm, so the
byte-id gate could not catch it (#263 class) — the fix converges on the
runtime oracle.

Mirror the arg-side closure #271 landed: both arms (≤24B @retscr and
>24B sret) now funnel N_ARRLIT through the literal element fill and
N_DOT/N_INDEX/deref through aggarg_srcaddr + the #265/#268 whole-
aggregate copy. Type-agnostic, so struct AND array returns are closed.
A close-by-construction loud-stop (rule 7) guards any future unhandled
aggregate source from reaching the scalar default.

Closes the callee-half of (b)/(c) and the addressable siblings. The
g = mk() global-receive caller-half is commit-2.

949_aggret_source_run pins the class: array-literal / N_DOT / N_INDEX /
deref / named-ident control / >24B-sret-deref / struct-field / struct-
deref, each summing all members (full readback) with per-row byte-id.

Makefile

@@ -406,6 +406,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_structlit_arrfield_run \
 	  $(BIN)/test_arraytoslice_run \
 	  $(BIN)/test_valstruct_subsize_run \
+	  $(BIN)/test_aggret_source_run \
 	  $(BIN)/test_continue_run \
 	  $(BIN)/test_callret_unsigned_arith_run \
 	  $(BIN)/test_sar_shr_run \
@@ -1621,6 +1622,11 @@ $(BIN)/test_valstruct_subsize_run: test/wcc/949_valstruct_subsize_run.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_aggret_source_run: test/wcc/949_aggret_source_run.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_continue_run: test/wcc/911_continue_run.c $(BIN)/ww \
 		$(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)