test: migrate 802_lenidx to @test + runww rejects, retire C twin (fold-3)

#19 len/index family: value rows -> test/lang/lenidx_test.ww (primitive-only asserts), 3 reject (callres/sliceexpr/strlit) -> runww //ww:error dual-stage carriers. byteid floor 52->53.

test/wcc/data/lenidx_callres/case.ww

@@ -0,0 +1,13 @@
+//ww:error "len() operand shape not place-resolvable (rule-7)"
+// Lifted from test/wcc/802_lenidx_run.c reject row "reject_callres"
+// (#10/#41/rule-7): len() over a CALL RESULT (`mk()`) is not a
+// place-resolvable operand and MUST loud-fail in both stages, not silently
+// return the data pointer as the length. The substring is the shared
+// diagnostic body — byte-identical on both stages (no file:line, no cstage
+// "ww: " prefix — both differ cs vs ww).
+package main;
+fn mk() []i64 = {
+	let xs: []i64 = [10, 20, 30];
+	return xs;
+};
+export fn main() i32 = { return len(mk()): i32; };