perf: next() stops scanning at the first available proxy

T3-18: _next_from_list scanned and collected every available proxy, then used
only the first (8.6 ms per call at 10k proxies). break after the first; the
selection order and log levels are identical (verified over full cycles with
burns, dead proxies, cooldown and the all-cooling fallthrough), ~1.8 us/call
at any pool size.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/aioproxies/manager.py

@@ -173,6 +173,8 @@ class AioProxies:
             if self._available(state["timeout"], now):
                 fine.append(idx)
                 self._clear_stale_burn(state)
+                # only the first available proxy is used; scanning on is O(pool) per call
+                break
 
         if fine:
             chosen = fine[0]