[backryun]

Part of #8484. Three unnecessary type erasures, three diagnostics, zero new.

Root cause

resolveStepDetailConfig() forced HeadroomConfig, SessionDedupConfig, and CcrConfig
through Record<string, unknown> assertions even though its only consumer immediately spreads the
returned detail object into stepConfig. These interfaces intentionally have specific fields and
no string index signature, so the assertions triggered TS2352.

This removes the explicit Record return annotation and the three assertions, allowing the
existing detail-config union to flow into the spread operation. Returned values and pipeline
behavior are unchanged.

Diagnostics

Measured independently against release/v3.8.49 at 0eeb8f45c:

• TypeScript 6 open-sse diagnostics: 150 → 147
• TypeScript 7.0.2 native diagnostics: 149 → 146
• Full line-number-agnostic error-set diff: 3 fixed, 0 new

With #8809, #8810, and #8811 included, the cumulative projection is
TS6 137 / TS7 136.

Validation

• npm run typecheck:core
• Compression detail persistence, Headroom merge, and stacked-pipeline integration tests —
  14/14 passed
• ESLint on open-sse/services/compression/stepDetailConfig.ts
• npm run check:file-size
• Prettier and git diff --check

No new runtime test was added because the returned objects and merge order are unchanged, while
the existing focused tests cover all three persisted detail sections and the stacked-pipeline
consumer.