feat(scoring): sync hardening — staleness gate + robust constant parser

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Found in the 2026-06-17 scoring-sync audit.

Problem

1. No staleness gate on the live scoring snapshot (model.ts:103) — a failed/old refresh silently serves last-good (or DEFAULT) constants with no warning on the preview side (Pipeline B has UPSTREAM_STALE_MS, the scoring side does not).
2. Parser truncates floats/underscores/sci-notation (model.ts:110 regex) — 1_000_000→1, 1e-9→1; harmless today but will misparse any newly-modeled constant and pollutes the unmodeled-list.

Deliverable

• Add a staleness flag (mirror UPSTREAM_STALE_MS) so previews can warn when constants are stale
• Fix the parser: strip underscores, support e-notation

Outcome

Stale/failed upstream sync is visible, not silent; parser is correct for all upstream numeric forms. Milestone M1.