feat(scoring): support exponential saturation previews

[oktofeesh1]

Summary

• update private preview model detection for saturation-era constants
• use saturation math when the active snapshot is saturation-based
• keep density-era snapshots working for historical data

What changed

• prefers saturation indicators when mixed constants are present
• records a warning for mixed density/saturation fixtures
• adds focused preview tests for saturation, mixed constants, old density fixtures, and public text hygiene
• keeps readiness integration fixtures current so freshness SLO assertions remain deterministic in CI

Why

• private preview metadata should track the active upstream scoring model instead of falling back to density-era assumptions.

Validation

• git diff --check
• npx tsc --noEmit
• npx vitest run test/unit/scoring.test.ts
• npm run test:coverage
• GitHub validate, Contributor trust, and Superagent Security Scan pass on the latest head

Closes #89

[chatgpt-codex-connector]

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[JSONbored]

@oktofeesh1 this is merge-ready from my pass.

• Deterministic: check rollup is green and merge-tree against current origin/main is clean.
• Maintainer judgment: the implementation now uses fetched model constants before fallback defaults and keeps the fallback path explicit.
• Validation: existing unit/integration coverage exercises the preview math and fallback behavior.