feat: Phase B fip_id liquid-breadth research tooling

Add research-only snapshot extender, PIT dollar-volume mask for signal IC,
rank-only harness path, fingerprint+breadth runner, and docs. Fingerprint
reproduced IC -0.045 / t -2.91 on prod.sqlite. No production gate/schedule changes.

reports/fip-breadth-20260718-194828.json

@@ -0,0 +1,21 @@
+{
+  "generated_at": "2026-07-18T19:48:28.127710",
+  "liquid_breadth_top_n": 1500,
+  "liquid_min_price": 5.0,
+  "fingerprint": {
+    "signal": "fip_id",
+    "weeks": 35,
+    "avg_cross_section": 497.7,
+    "mean_ic": -0.045,
+    "ic_t_stat": -2.91,
+    "ic_positive_pct": 25.7,
+    "mean_quintile_spread": -0.0168,
+    "reliable": true,
+    "pass": true,
+    "expected_ic": -0.045,
+    "expected_t": -2.9
+  },
+  "breadth": null,
+  "verdict": null,
+  "fingerprint_report_path": "reports\\fip-breadth-20260718-194828-fingerprint.json"
+}