Fix findCounter infinite loop; corpus filter skips chunks past 256 MB

lastIndexOf('(', -1) searches from 0 again, so a subject starting with a "("
that isn't a counter, without a quote, looped forever. The corpus filter hit
it at 11:23 on Oct 24: the coordinator (and corpus:filter) spun at 100% CPU
for 10 hours and no range finished. The Rust find_counter stops at 0 already.

A chunk inflating past V8's longest string can't be filtered as text; such a
chunk is now stored unfiltered instead of throwing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/edge/scan.ts

@@ -47,6 +47,8 @@ interface Counter {
 function findCounter(subject: string, after: number): Counter | null {
   let open = subject.length
   while (true) {
+    // lastIndexOf('(', -1) searches from 0 again: a "(" at the start that isn't a counter looped forever.
+    if (open === 0) return null
     open = subject.lastIndexOf('(', open - 1)
     if (open <= after) return null
     let i = open + 1