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_strlit/case.ww

@@ -0,0 +1,9 @@
+//ww:error "len() operand shape not place-resolvable (rule-7)"
+// Lifted from test/wcc/802_lenidx_run.c reject row "reject_strlit"
+// (#10/#41/rule-7): len() over a string LITERAL 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;
+export fn main() i32 = { return len("abc"): i32; };