feat: signal math upgrade — probabilistic, regime-aware scoring pipeline

Implement full probabilistic signal processing pipeline gated behind
probabilistic_scoring_enabled feature flag in risk_configs:

- Bayesian log-likelihood accumulator with Beta posterior and entropy
- Regime detector (trend-following, panic, mean-reversion, uncertainty)
- Source accuracy tracker with per-source historical prediction accuracy
- Sigmoid confidence gate replacing binary gate
- Information gain surprise weighting for rare events
- Adaptive recency decay with event-specific half-lives
- Regime multiplier replacing market context multiplier
- Weighted disagreement entropy for contradiction detection
- Multiplicative macro exposure with conditional integration
- Graph-distance attenuated competitive signal propagation
- Exponentially weighted momentum with volatility scaling
- Expected value recommendation gate

All changes backward-compatible: flag=false preserves exact current behavior.
New outputs stored in existing JSONB columns (no schema changes except
source_accuracy table via migration 034).

Tests: 26 property-based tests (14 correctness properties), 99 unit tests,
1789 total tests passing with zero regressions.

infra/migrations/034_source_accuracy.sql

@@ -0,0 +1,18 @@
+-- Source accuracy tracking table for historical prediction accuracy per source.
+--
+-- Stores per-source accuracy metrics (fraction of correct directional calls)
+-- used by the probabilistic scoring pipeline to weight source credibility.
+-- See Requirement 4.5: source accuracy metrics stored with source identifier,
+-- accuracy ratio, sample count, and last updated timestamp.
+
+CREATE TABLE IF NOT EXISTS source_accuracy (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    source_id VARCHAR(200) NOT NULL,
+    accuracy_ratio FLOAT NOT NULL DEFAULT 0.5,
+    sample_count INTEGER NOT NULL DEFAULT 0,
+    last_updated TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    UNIQUE(source_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_source_accuracy_source ON source_accuracy(source_id);