feat(signal_v2-phase4): add emit/skip logging to signal_generator

logger was declared but unused. Operational visibility was zero —
trader debugging 'why no signal?' had to step through code mentally.

- INFO on emit: '[signal emit] 005930 buy conf=0.823 rank=3' / sell with reason
- DEBUG on each skip path: same-cycle sell, hard gate, low confidence,
  dedup 24h (buy and sell)

Per final reviewer recommendation. 56 tests still pass.

signal_v2/signal_generator.py

@@ -32,16 +32,22 @@ def _evaluate_buy_signals(state, dedup, settings) -> None:
     for ticker, name, rank in candidates:
         existing = state.signals.get(ticker)
         if existing is not None and existing.get("action") == "sell":
+            logger.debug("buy %s skipped: same-cycle sell precedence", ticker)
             continue
         if not _check_buy_hard_gate(state, ticker, settings):
+            logger.debug("buy %s skipped: hard gate failed", ticker)
             continue
         confidence = _compute_buy_confidence(state, ticker, rank)
         if confidence <= settings.confidence_threshold:
+            logger.debug("buy %s skipped: confidence %.3f <= %.3f",
+                         ticker, confidence, settings.confidence_threshold)
             continue
         if dedup.is_recent(ticker, "buy", within_hours=24):
+            logger.debug("buy %s skipped: dedup 24h", ticker)
             continue
         state.signals[ticker] = _build_buy_signal(state, ticker, name, rank, confidence)
         dedup.record(ticker, "buy", confidence=confidence)
+        logger.info("signal emit %s buy conf=%.3f rank=%s", ticker, confidence, rank)
 
 
 def _buy_candidates(state) -> list[tuple[str, str, int | None]]:
@@ -122,9 +128,13 @@ def _evaluate_sell_signals(state, dedup, settings) -> None:
         if sell is None:
             continue
         if dedup.is_recent(ticker, "sell", within_hours=24):
+            logger.debug("sell %s skipped: dedup 24h", ticker)
             continue
         state.signals[ticker] = sell
         dedup.record(ticker, "sell", confidence=sell["confidence_webai"])
+        logger.info("signal emit %s sell conf=%.3f reason=%s",
+                    ticker, sell["confidence_webai"],
+                    sell.get("context", {}).get("sell_reason"))
 
 
 def _try_stop_loss(state, holding: dict, settings) -> dict | None: