automata: fix leftmost-first miss in the reverse suffix optimization

[RECHE23]

The reverse suffix optimization can skip a match that starts earlier than the matches it reports, violating leftmost-first semantics.

Repro (1.12.4, default config)

let re = regex::Regex::new("a|.aa").unwrap();
let spans: Vec<_> = re.find_iter("-#aa").map(|m| (m.start(), m.end())).collect();
assert_eq!(spans, vec![(1, 4)]);   // FAILS today: [(2, 3), (3, 4)]

[1,4) (".aa" matching #aa) is a match and starts leftmost. (Original fuzz find: \0|.\0\0 on "\0\0\0\0\n\x0b\0\0\0", missing (5,8); minimized by hand.)

Root cause

With RUST_LOG=trace: suffixes extract as Seq[E("a"), I("aa")], the LCS prefilter is memchr('a'), and ReverseSuffix is engaged. Candidates are processed in order of match end, and the first reverse+forward confirmed match is yielded. That is only correct when the leftmost-starting match also has the smallest candidate end at/after the resume point. Here branch 1's entire match (E("a")) is a proper suffix of branch 2's suffix ("aa"): a branch-1 match nests inside the tail of an overlapping branch-2 match that ends later but starts earlier. Candidate end 3 confirms (2,3) and is yielded; the true leftmost [1,4) (candidate end 4) is then behind the (correct in itself) anti-quadratic min_start clamp, so the miss is permanent.

The fix

Conservative: decline the optimization in ReverseSuffix::new when any exact suffix literal is a proper suffix of another suffix literal — the precise nesting that breaks the end-order ⇒ start-order assumption — falling back to the core engine. Common beneficiaries keep the optimization (single-branch suffixes, all-inexact seqs such as \w+@\w+, non-nested alternations like foo|bar).

A complete fix that keeps the optimization for these patterns would need candidate look-ahead across ends before yielding when overlap is possible; happy to explore that as a follow-up if you'd prefer.

Validation

• New regression test in tests/regression_fuzz.rs (fails before, passes after).
• cargo test --test integration (66) and cargo test -p regex-automata --lib (137): pass.

Found by a differential fuzzer (REAL-regex vs this crate).

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