test: migrate 948_idx_compound to @test + runww rejects, retire C twin (fold-3)

#133 indexed-scalar compound-assign. The 21 rows split by observability:
18 value rows -> test/lang/idx_compound_test.ww (one @test fn each,
primitive-only asserts, slot-poison + neighbor-unchanged read-back);
3 reject rows -> test/wcc/data/idx_compound_{float_indexed,str_indexed,
chained_ptr_float}/case.ww as //ww:error, asserting BOTH stages reject on
the shared diagnostic body (cstage's leading prefix excluded). The C twin
is retired; its coverage is a strict superset of the original rows.

First reject-bearing fold-3 migration -- validates the runww dual-stage
home end to end. LANGBYTEID_EXPECTED_MIN 49 -> 50.

Makefile

@@ -548,7 +548,6 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_checked_run \
 	  $(BIN)/test_floatarr_run \
 	  $(BIN)/test_inferred_scalar_global_run \
-	  $(BIN)/test_idx_compound_run \
 	  $(BIN)/test_dotbase_addr_slice_run \
 	  $(BIN)/test_globalslice_arg_run \
 	  $(BIN)/test_structlit_arrfield_run \
@@ -3055,11 +3054,6 @@ $(BIN)/test_inferred_scalar_global_run: test/wcc/947_inferred_scalar_global_run.
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
-$(BIN)/test_idx_compound_run: test/wcc/948_idx_compound_run.c $(BIN)/ww \
-		$(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
-		$(LIB)/libwwrt.a | $(BIN)
-	$(CC) $(CFLAGS) -o $@ $<
-
 $(BIN)/test_dotbase_addr_slice_run: test/wcc/949_dotbase_addr_slice_run.c \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
@@ -3316,7 +3310,7 @@ test-lang: all
 LANGBYTEID_DIR		= $(OUT)/langbyteid
 LANGBYTEID_FILES	= $(wildcard test/lang/*_test.ww)
 LANGBYTEID_VERB		= test -c
-LANGBYTEID_EXPECTED_MIN	= 49
+LANGBYTEID_EXPECTED_MIN	= 50
 $(if $(LANGBYTEID_FILES),,$(error test-lang-byteid: empty corpus))
 test-lang-byteid: all
 	@set -e; \