feat: add ledger digest layer to specflow-watch review section (#183)

[skr19930617]

Summary

• Adds a ledger-backed digest layer to the specflow-watch Review section, rendered below the existing snapshot progress lines without altering snapshot behavior.
• New tolerant readers for review-ledger-design.json / review-ledger.json with full-schema validation, decision parity check, and tighter ReviewFinding validation (id/title/severity/status required).
• Family selection (selectActiveReviewLedger) mirrors the snapshot family rule; digest is hidden for non-review phases.
• Digest renders Decision / Findings counts / open-severity breakdown / top-3 unresolved findings ranked by severity (HIGH or CRITICAL → MEDIUM → LOW) → latest_round DESC → id ASC.
• Below 80 columns, the findings list collapses and other digest lines truncate with an ellipsis (…) instead of wrapping.
• Watcher remains strictly read-only — import-graph guard unchanged; no new orchestration imports, no new observation events.
• Phase 1 omits the Latest summary: line per the spec scenario "Latest round summary missing elides the line"; a Phase 2 follow-up can add a persisted narrative source.

Test plan

• npm test — 953 tests pass
• Reader tests cover absent / unreadable / malformed / empty round_summaries / decision-parity violation / finding-missing-required-field
• Model tests cover hidden / placeholder / warning / ok states, severity aggregation including critical → HIGH, ranking ties, summaryState absent
• Renderer tests cover wide-terminal full digest, narrow-terminal collapse + ellipsis, ledger-only (snapshot absent) still renders digest, digest hidden on non-review phases
• Integration tests cover initial digest render and redraw after on-disk ledger update
• Import-graph guard verifies no orchestration mutators leaked into the watch entrypoint

Issue

Closes #183