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.

test/wcc/data/idx_compound_float_indexed/case.ww

@@ -0,0 +1,13 @@
+//ww:error "indexed-lvalue compound on float element not wired (#133/rule-7)"
+// Lifted from test/wcc/948_idx_compound_run.c reject row "he_float_indexed"
+// (#133-expanded rule-7): a compound assign on an indexed FLOAT element is
+// unwired in both stages and MUST loud-fail, not silently fall through to the
+// default break (silent miscompile). The substring is the FULL shared
+// diagnostic body — byte-identical on both stages (no file:line prefix, no
+// cstage "ww: " prefix — both differ cs vs ww).
+package main;
+export fn main() i32 = {
+	let a: [4]f64 = [1.0, 2.0, 3.0, 4.0];
+	a[0] += 1.5;
+	return 0;
+};