docs(sor): follow-up plan — RQ2-P3 mechanism prereg + RQ3 run-brief + unique-bridge artifact note

RQ2-P3 mechanism study (NEW prereg, approved-path/params-locked, not yet frozen):
pool-based willing-bridge instrument, concentration swept as a manipulated IV
(B∈{2,4,8}×α∈{0,1,2}), two-sided/direction-agnostic, run-level cluster bootstrap.
RQ3 companion run-brief (execution-freeze for the already-frozen RQ3): agent=qwen2.5:3b
local Ollama, churn kill_prob=30/steps=20, baselines + Holm-7 completion pinned.
note-unique-bridge-artifact.md: on-record analysis that the lead RQ2-P1 shrink is plausibly
a fresh-bridge-per-circuit artifact (unique exit-signature -> singleton set -> H~0), and that
realistic bridge reuse (a mix) may raise anonymity -> the mechanism study may qualify the lead
RQ2 headline. No edit to the frozen prereg or committed lead paper.

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

docs/note-unique-bridge-artifact.md

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+# Note — the RQ2-P1 "shrink" is plausibly a unique-bridge instrument artifact
+
+**Status:** on-record analysis note, 2026-07-21. Does **not** edit the frozen prereg or the
+committed lead paper; it documents a mechanism concern for the record and motivates the
+`rq2p3-mechanism-prereg.md` follow-up. Author: overseer, post-lead-paper review.
+
+## The finding
+
+The lead paper (RQ2-P1) reports federation **shrinks** the per-circuit anonymity set
+(ΔH = −0.96 bits pooled; −3.63 bits bridge-federated-only, exploratory). Tracing the ratified
+posterior mechanically shows this shrink is **largely forced by the instrument**, not by a
+substantive funnelling phenomenon:
+
+1. The adversary's observation is `exit_signature = (exit_house, bridge_label)`
+   (`confirm_load_rq2.py:exit_signature`).
+2. The anonymity set for circuit *i* is the distinct **entry** nodes among circuits sharing
+   *i*'s signature (`observation_consistent_sizes`): `m_i = |{entry(j) : sig(j) == sig(i)}|`.
+3. The bridge-federated instrument assigns a **fresh bridge per circuit seed**
+   (`assembler.py:_bridge_label(seed)`), so **every circuit's `bridge_label` is unique** ⇒
+   every `exit_signature` is unique ⇒ each group has size 1 ⇒ **`m_i = 1` ⇒ `H_i ≈ 0`.**
+
+So bridge-federated collapses to near-zero entropy **because each circuit was handed its own
+bridge**, making its signature a unique identifier. That is the *same* degeneracy that made
+RQ2-P3 untestable (zero-variance concentration) — here it manifests as an artificially low H
+that drives ΔH negative.
+
+## Why this likely inverts under realistic bridge reuse
+
+Introduce a finite shared bridge **pool**: many circuits share a bridge ⇒ shared signature ⇒
+`m_i` = distinct entries across all of them ⇒ **larger set ⇒ higher H**. Under this posterior
+a shared bridge behaves as a **mix**: more concentration plausibly *raises* anonymity — the
+opposite of the naive "funnelling shrinks the set" story. The mechanism study
+(`rq2p3-mechanism-prereg.md`) tests this **two-sided**; the mechanical prediction is mix
+(ρ > 0), the naive prediction is funnel (ρ < 0).
+
+## Consistency check against observed numbers
+
+- Directory-federated cell (no bridge hop → `bridge_label = None`, shared signature):
+  observed `rq2_anonymity_entropy_bits = 5.6439 = log₂(50)` — **maximal** entropy, all 50
+  senders in the set. Confirms shared signatures give large sets.
+- Bridge-federated (unique bridge per circuit): H ≈ 0, consistent with the exploratory
+  ΔH = −3.63. The pooled −0.96 is the average of a high-H directory arm and a ≈0-H bridge arm.
+
+## Integrity status
+
+- The lead paper is **not wrong or dishonest**: §7 already flags RQ2 as conditional on the
+  posterior construction and RQ2-P3 as an unresolved mechanism. This note **sharpens** that
+  caveat from "conditional" to "the specific shrink magnitude is mechanically forced by the
+  fresh-bridge instrument."
+- **No silent edit** to the committed paper or frozen prereg. If the mechanism study confirms
+  mix (ρ > 0), the honest outcome is a companion result that **qualifies/corrects** the lead
+  RQ2-P1 headline — reported openly, per the null-results-are-results rail.
+- This strengthens the body of work: it is exactly the kind of artifact a pre-registered,
+  frozen-detector method is supposed to expose rather than bury.