Pin dose-response cohort to period 3 (P1); harden narrative to close-race shifts

CI re-review P1: bench_dose_response.py inherited the CDiD generator's
default cohort [2], not the documented period 3. The fallback that
would have set first_treat=3 never ran (generator already populates
first_treat), so the committed baselines measured a different cohort
onset than the scenario doc. The binarized DiD phase also hardcoded
post >= 3, which further desynced it from the actual CDiD treatment
start under the default DGP.

Fix:
- Pin the generator to cohort_periods=[3] so the DGP matches the docs.
- Assert exactly one positive first_treat after generation; future
  DGP changes that break the single-cohort contract will fail loudly
  instead of drifting silently.
- Binarized phase now derives its post cutoff from the actual
  first_treat in the data, not a hardcoded period number. No
  opportunity to desync from the CDiD fits above.
- Regenerated dose-response baselines for both backends.

Structural narrative hardening:

Prior CI rounds have repeatedly re-flagged the same drift pattern:
the staggered campaign and reversible dCDH narratives make phase-
order claims at close-race cells (staggered Rust medium, dCDH at
this shape) that can flip on rerun because the two contenders are
within a few percentage points of each other. The underlying ranking
is not the right level of abstraction for narrative; the phase-share
table is. This commit rewrites both narratives to describe the
aggregate share pattern and defer per-cell ordering to the
generator-produced table. Scaling finding #2 and hotspot table row
#2 get the same treatment. Net effect: narrative claims are now
robust to rerun noise at close-race cells.

Still measurement only. No changes under diff_diff/ or rust/.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

benchmarks/speed_review/baselines/brand_awareness_survey_large_python.json

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benchmarks/speed_review/baselines/brand_awareness_survey_large_rust.json

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benchmarks/speed_review/baselines/brand_awareness_survey_medium_python.json

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benchmarks/speed_review/baselines/brand_awareness_survey_medium_rust.json

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benchmarks/speed_review/baselines/brand_awareness_survey_small_python.json

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