chore(scoring): single-source fallbacks + drop dead density branch

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Found in the 2026-06-17 scoring-sync audit (low-severity cleanup).

Problem

1. Fallback literals (25, 0.8, 0.15, 1.33, 1.66, 0.4...) are duplicated across model.ts DEFAULT_SCORING_CONSTANTS and every constant() call in preview.ts — two sources of truth.
2. Density-model constants (MIN_TOKEN_SCORE_FOR_BASE_SCORE, MAX_CODE_DENSITY_MULTIPLIER) + the density branch (preview.ts:301-308) are dead while upstream is on the saturation model — silent-fallback surface.

Deliverable

• Derive preview fallbacks from DEFAULT_SCORING_CONSTANTS (single source)
• Confirm upstream is committed to the saturation model; if density is dead, remove the branch

Outcome

One source of truth for fallbacks; smaller silent-drift surface. Milestone M1.