Fix of the v0.3 review findings:
- No silent callback drop — PUSH4-found selectors without a PUSH2 dispatcher entry now fall back to a labeled whole-contract scan ("isolation unverified") instead of being discarded
- No segment truncation — extract_callback_segment no longer cuts a handler at an internal REVERT/INVALID branch-exit followed by real code
- Clippy-clean — all cargo clippy -D warnings errors fixed
- +4 regression tests (7 total)