feat(strategy): GNews guardrail — clamp news-only shifts to prior±0.25

Post-mortem NVIDIA 631181: one uncorroborated high-weight signal (legacy
Manifold 0.13 at weight 0.6) flipped a 0.845 market to 0.431 and lost.
With Manifold observational-only and macro signals gated behind
is_non_price, GNews (weight 1.5) is the only live signal able to move
politics markets 20-30 pp against the order-book consensus.  This adds a
catastrophic fuse, not a fine calibration:

- apply_news_guardrail(): when |news_lo| >= NEWS_MATERIAL_LOGODDS_THRESHOLD
  (0.10) and every other signal (fg, mom, btc_dom, mfld) is below it,
  clamp the posterior to prior ± MAX_NEWS_ONLY_PROB_SHIFT (0.25).  Any
  corroborating material signal disables the clamp.  Config via env
  (NEWS_GUARDRAIL_ENABLED=true by default).
- edge_gross/edge_net computed from the clamped posterior; raw_final_prob
  preserved in reasoning (persisted via trades.reasoning — no schema
  migration) and in the NEWS_MATERIAL log line.
- guardrail_changed_trade_decision: raw edge crossed the regime gate but
  the clamped edge no longer does (fuse prevented a trade).  Note: with
  the default 0.25 band the clamped edge_net is 0.21, above every regime
  minimum, so the flag only fires with a tighter configured band.
- Observability gated on materiality: NEWS_MATERIAL per-market line and a
  compact NEWS SUMMARY cycle line, only when with_news > 0 — no flood
  from the ~145 news-less markets per cycle.
- 9 deterministic tests (extreme clamp, in-band passthrough, corroboration,
  inclusive threshold, disabled, changed_decision).

No changes to NEWS_LOGODDS_WEIGHT, Manifold flags, edge thresholds,
sizing, payout, resolution, or historical trades.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bot/main.py

@@ -277,6 +277,20 @@ async def run_trading_loop(
                 manifold_summary,
             )
 
+            # NEWS SUMMARY — one compact line, only on cycles where at least
+            # one market had a material GNews contribution (never an empty
+            # section on news-less cycles).
+            if stats["news_with_material"] > 0:
+                log.info(
+                    "NEWS SUMMARY | with_news=%d | avg_shift=%+.2f | "
+                    "max_shift=%+.2f | guardrail_applied=%d | changed_decisions=%d",
+                    stats["news_with_material"],
+                    stats["news_avg_shift"],
+                    stats["news_max_shift"],
+                    stats["news_guardrail_applied"],
+                    stats["news_changed_decisions"],
+                )
+
             # 9. Update daily metrics
             await metrics.update_daily_summary()