RQ2-P3: bridge-federated-pool mechanism instrument + synthetic tests + calibration gate

Build the funnelling-mechanism instrument for the RQ2-P3 follow-up: a new
`bridge-federated-pool` topology drawing each circuit's bridge from a finite
willing-bridge pool (size B) under Zipf willingness skew (alpha), so bridge
concentration genuinely varies as a manipulated IV. Defensive measurement only:
synthetic seeds, no engine, no traffic, no confirmatory record read.

- assembler.py: add `bridge-federated-pool` branch + zipf_weights/weighted_draw
  helpers; existing lead `bridge-federated` branch left untouched and
  bit-reproducible (fresh bridge per seed).
- battery.py: enumerate_rq2p3_cells() = B{2,4,8} x alpha{0,1,2} + anchor B=50,
  alpha=0 (separate fn; frozen 6-cell lead lattice unmutated).
- analysis/rq2p3_calibration.py: DRY synthetic-only §7 calibration gate; no
  confirmatory data read. confirm_load_rq2.py/confirm.py/stats.py unchanged.
- tests/test_sor_rq2p3_pool.py: 9 synthetic tests (helpers + pool reuse/variance
  + lead branch reproducibility).

Calibration gate RAN: items 3 (monotonicity) & 4 (entropy) PASS; items 1 & 2 do
NOT match their naive-funnel wording because the ratified posterior yields a MIX
(rho>0 across the whole sweep incl anchor rho=+0.838; B=1 -> H=2.54 bits HIGH) --
the note-unique-bridge-artifact.md prediction, surfaced early. HARD HOLD: no
confirmatory battery, no prereg freeze; NEEDS-OPERATOR banner raised for the §7
item-1/2 re-word decision. Lead prereg SHA f22331a72e... untouched; worktree-only.

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

cmd_chat/sor/battery.py

@@ -92,6 +92,40 @@ def enumerate_cells() -> List[Cell]:
     return cells
 
 
+def enumerate_rq2p3_cells() -> List[Cell]:
+    """The RQ2-P3 funnelling-mechanism sweep cells (`rq2p3-mechanism-prereg.md` §4),
+    kept **separate** from the frozen lead lattice: `enumerate_cells()` above is left
+    byte-identical so every lead-pipeline caller (`plan_runs`, `battery_schedule`,
+    `assembler_dry_check`, `collect_rq2_p1_arms`) stays bit-reproducible.
+
+    These use the new `bridge-federated-pool` topology (finite willing-bridge pool of
+    size ``B`` under a Zipf willingness skew ``alpha``) so concentration genuinely
+    varies. Tagged ``rq = "RQ2P3"`` so they are distinct from the frozen RQ1/RQ2
+    cells and are skipped by the lead RQ2 collectors (which filter ``rq == "RQ2"``).
+
+    9 sweep cells: ``B ∈ {2,4,8} × alpha ∈ {0, 1.0, 2.0}``, plus the ``B=50, alpha=0``
+    **calibration anchor** that reproduces the lead RQ2-P3 degeneracy (§7 gate item 1).
+    """
+    cells: List[Cell] = []
+    for b in (2, 4, 8):
+        for alpha in (0.0, 1.0, 2.0):
+            cells.append(Cell(
+                "RQ2P3",
+                f"RQ2P3/topo=bridge-federated-pool/selector=static/B={b}/alpha={alpha}",
+                {"bridge": "off", "topology": "bridge-federated-pool",
+                 "selector": "static", "pool_B": str(b), "pool_alpha": str(alpha)},
+                False,
+            ))
+    cells.append(Cell(
+        "RQ2P3",
+        "RQ2P3/topo=bridge-federated-pool/selector=static/B=50/alpha=0.0",
+        {"bridge": "off", "topology": "bridge-federated-pool",
+         "selector": "static", "pool_B": "50", "pool_alpha": "0.0"},
+        False,
+    ))
+    return cells
+
+
 def declared_na_cells() -> List[Cell]:
     """N/A cells declared at the design (not run, not dropped ad hoc) — padding is
     only defined for bridge-on, so bridge-off+padding is N/A (§2)."""