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>

app/services/regime_monitor_service.py

@@ -139,8 +139,10 @@ P3_DRAWDOWN_ANCHORS = (
 # why the crossing gets a floor of 20 rather than starting at 0. Calibrated to
 # sit alongside P3 rather than swamp it -- the 200-DMA lags, so a 20% drawdown
 # typically coincides with ~10% below the average, where this reads ~61 against
-# P3's ~59. Over the 408 sessions to 2026-07-24 P1 is the sole price argmax on
-# 17 of them (4.2%), so it informs the pillar without owning it.
+# P3's ~59. On the population the P1_SCORE_CAP rule actually names -- sessions
+# with State >= 40 -- P1 is the sole price argmax on 17 of 47 (36.2%), against
+# P2's 16 and P3's 14, so it informs the pillar without owning it and no cap
+# was needed.
 P1_TREND_BREAK_ANCHORS = (
     (0.0, 20.0), (3.0, 35.0), (8.0, 55.0), (15.0, 75.0), (25.0, 100.0),
 )