fix: composite score renormalization + CCB weighted sampling

Bug 1: compute_composite_score() was unreachable at 0.65 threshold
because chain_recall/symbol_recall defaulted to 0 when unavailable,
capping max score at 0.70. Fix: None signals "not measured" and weight
is redistributed proportionally to available components.

Bug 2: ccb_weighted_sample() duplicated task objects for 2x boost then
deduped by identity, so the boost never worked. Rewrote to boost
effective stratum allocation sizes and prefer CCB tasks within each
stratum.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sjarmak

configs/variance_reruns/pass2_local.json

@@ -0,0 +1,48 @@
+{
+  "metadata": {
+    "title": "Pass 2 local: 2 tasks needing 2nd pass",
+    "generated_date": "2026-03-02",
+    "total_tasks": 2
+  },
+  "methodology": {
+    "sdlc_suites": [
+      "ccb_debug"
+    ]
+  },
+  "statistics": {
+    "total_tasks": 2,
+    "per_suite": {
+      "ccb_debug": 2
+    }
+  },
+  "tasks": [
+    {
+      "task_id": "teleport-ssh-regression-prove-001",
+      "benchmark": "ccb_debug",
+      "task_dir": "ccb_debug/teleport-ssh-regression-prove-001",
+      "language": "go",
+      "difficulty": "hard",
+      "current_bl_runs": 1,
+      "current_mcp_runs": 3,
+      "current_paired": 1,
+      "runs_needed": 2,
+      "sdlc_phase": "debug",
+      "repo": "gravitational/teleport",
+      "mcp_benefit_score": 0.75
+    },
+    {
+      "task_id": "tutanota-search-regression-prove-001",
+      "benchmark": "ccb_debug",
+      "task_dir": "ccb_debug/tutanota-search-regression-prove-001",
+      "language": "typescript",
+      "difficulty": "hard",
+      "current_bl_runs": 3,
+      "current_mcp_runs": 1,
+      "current_paired": 1,
+      "runs_needed": 2,
+      "sdlc_phase": "debug",
+      "repo": "tutanota/tutanota",
+      "mcp_benefit_score": 0.75
+    }
+  ]
+}