Early-exit validation: stop once enough valid proxies found

- validate_proxies: target= param — once N valid found, remaining
  tasks skip the HTTP call and drain immediately (no wasted time)
- _build_pool: target = max(min_pool_size, chain_length * 3)
- Settings: min_pool_size (default 20), max_candidates default 400->200
- UI: Stop after N valid / Max candidates fields in Settings tab
- README: updated settings table
- 1 new test for early-exit with unreachable target

Made-with: Cursor

README.md

@@ -128,7 +128,8 @@ Hits config sanitization, fetcher smoke, validator timeouts, and a live `get_dir
 | Health check | `180` s | How often the exit **gets re-proven** |
 | Pool refresh | `1800` s | How often lists get **re-fetched** and re-tested |
 | Concurrency | `64` | How hard you **spam** validation |
-| Max candidates | `400` | Cap per cycle before testing |
+| Stop after N valid | `20` | **Early-exit** — stop testing as soon as N good proxies found |
+| Max candidates | `200` | Cap per cycle (shuffled, so random sample) |
 | Timeout | `12` s | Per-proxy **patience** |
 | Kill-switch | ON | **All-or-nothing** outbound (when Admin + firewall engaged) |