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

proxy_chain_manager/config.py

@@ -126,7 +126,8 @@ class Settings:
 
     # ── validation ────────────────────────────────────────────────────────
     validation_concurrency: int = 64
-    max_candidates: int = 400
+    max_candidates: int = 200          # cap before validation (shuffled, so random sample)
+    min_pool_size: int = 20            # stop validating once this many good proxies found
     validation_timeout_seconds: float = 12.0
     prefer_elite: bool = False
 
@@ -204,7 +205,21 @@ def sanitize_settings(s: Settings) -> tuple[Settings, bool]:
             s.max_candidates = 10
             changed = True
     except (TypeError, ValueError):
-        s.max_candidates = 400
+        s.max_candidates = 200
+        changed = True
+    try:
+        mps = int(s.min_pool_size)
+        if mps < 3:
+            s.min_pool_size = 3
+            changed = True
+        elif mps > 500:
+            s.min_pool_size = 500
+            changed = True
+    except (TypeError, ValueError):
+        s.min_pool_size = 20
+        changed = True
+    if not hasattr(s, "min_pool_size"):
+        s.min_pool_size = 20
         changed = True
     try:
         vt = float(s.validation_timeout_seconds)