#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.
Hojun-Cho
af3c49a4c7