fix(research): require the v2 reproduction, and correct the P1-cap denominator

Two review findings, plus a lost-edit repair.

v2_reconstruction is now a required variant. It carries every published figure
the reproduction rests on (avg, p80, max, P3-pegged, W1-live), so a run without
it could emit a confident, non-provisional recommendation having checked nothing
against v2 at all -- while the methodology doc claims v2 and v3 are reproduced
first. The default invocation is now derived from REQUIRED_VARIANTS so the two
cannot drift, and a test asserts the default satisfies its own requirement.

The doc and the P1_TREND_BREAK_ANCHORS comment still justified skipping the
P1_SCORE_CAP with 17/408 = 4.2%, which is the all-session share and does not
evaluate the rule. The rule names sessions with State >= 40: 47 of them, P1 sole
argmax on 17 = 36.2%, against P2's 16 and P3's 14. Conclusion unchanged -- well
under the 80% trigger -- but the published rationale now states the metric that
actually decided it.

Root cause of that survival: the earlier correction WAS made, but in a script
that applied several substitutions and wrote the file once at the end. A later
substitution raised, so the successful edits were discarded with it. The
"Unlike P3 and V1 ... P3's do not" fix was lost the same way and is restored.

Also adds tests for the refusal paths themselves -- missing required variant,
unknown variant, custom window with no calendar anchor. They were verified by
hand last round but left unpinned, which is the same shape of problem as the
optional gates they exist to enforce. All return before any network call.

Deliberately not done, as not load-bearing: recording the oas400 variant's
missing-credit session count (the truncation conclusion rests on the
distribution mismatch, which is already recorded), and generalising
_pipeline_gates for arbitrary --end/--sessions windows.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/research/regime-monitor-v4.md

@@ -57,9 +57,11 @@ used to read 100.
 
 `max()` was **kept**. The defect was the step function feeding it, not the vote
 itself, and v3's "one capped vote for correlated reads" rationale still holds.
-Over the window P1 is the sole price argmax on 17 of 408 sessions (4.2%), so it
-informs the pillar without owning it — the `P1_SCORE_CAP` fallback considered
-during design was measured as unnecessary and not shipped.
+The `P1_SCORE_CAP` fallback drafted during design was to fire if P1 became the
+sole price argmax on **more than 80% of sessions with State ≥ 40** — i.e. if it
+had quietly become a second drawdown sensor. Measured on that population: 47
+qualifying sessions, P1 sole argmax on **17 of them (36.2%)**, against P2's 16
+and P3's 14. Well under the threshold, so the cap is not shipped.
 
 **The top State band moved 80 → 65.** See Calibration; this is the one change
 that is about the band rather than a sensor.
@@ -148,7 +150,7 @@ against P3's ~59.
 V1 reaches full scale at 55 rather than at 2020's ~82: anchoring the top at a
 once-in-a-generation print would make VIX 50 — a genuine crisis — read only ~70.
 The anchors encode the long-run distribution as constants, the same argument the
-credit level uses. Unlike P3 and V1, whose slopes ease off monotonically, P3's do
+credit level uses. Unlike P1 and V1, whose slopes ease off monotonically, P3's do
 not (2.5, 3.75, 3.125, 2.33, 1.83) — its gentle onset is intentional and the
 monotone-slope test excludes it.