Drop cluster="psu" from naive phase; tighten two narrative claims (P3)

CI re-review P3:

- bench_brand_awareness_survey.py "naive" phase was using
  cluster="psu", which is already a partial sampling-design correction
  - the SE-inflation comparison is more faithful to Tutorial 17 when
  the first phase is genuinely untreated-for-design. Removed the
  cluster argument.
- performance-plan.md narrative overreaches corrected:
  - Staggered campaign: at Rust medium SunAbraham is now the clearly
    leading phase (~1.7x ImputationDiD there), not "slightly edges
    out". Reworded to say ImputationDiD / SunAbraham are the top two
    at every scale but their order is not stable across backend and
    scale.
  - Reversible dCDH: split is not "~evenly under both backends" -
    Python is closer to 58/41 with the main fit leading, Rust is
    51/49 with the heterogeneity refit leading. Reworded to reflect
    the split per backend.

Regenerated the affected brand-awareness and campaign-staggered
baselines (the naive-fit change slightly reduces brand-awareness
chain totals and shifts phase-percentage shares). Tables in
performance-plan.md re-derived via gen_findings_tables.py.

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

@@ -2,47 +2,47 @@
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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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