feat: Scan Ledger — versioned scan history + trajectory UI

[New1Direction]

Summary

Adds Scan Ledger — a versioned history of every repo scan that surfaces each repo's trajectory, making the product's "evaluations compound" thesis real (previously saveRepo overwrote history on every re-scan).

• New snapshots IndexedDB store — one record per repo, additive v3→v4 migration (existing data untouched). A trimmed snapshot (health, fit, stars, flag titles, version, ts) is appended on every saveRepo, ring-buffer capped at 30/repo. An existing repo's first re-scan seeds the prior scan so its history isn't lost.
• New pure, fully-tested snapshots.js — toSnapshot / appendSnapshot / snapshotTrend / sparkline. FIT_ORDER is now exported from diff-analysis.js (instead of a 4th copy).
• UI: a health sparkline on library cards + a "History" strip on the Verdict tab (health/fit/flags over time). Both hidden until a repo has ≥2 scans; static under prefers-reduced-motion.
• Backup round-trip: snapshots join the envelope (BACKUP_VERSION → 2, each row clamped to 30 on import). Also fixes a pre-existing bug where decisions were silently omitted from backups.

Built subagent-driven from a committed spec + plan (docs/superpowers/specs|plans/2026-06-15-scan-ledger.*), each task TDD-gated. Two independent reviewers (spec-compliance + code-quality) ran over the diff; all flagged items fixed.

Test plan

• npm test — 752 passing (new snapshots / store / backup cases)
• npm run lint — 0 errors
• Independent spec + code-quality review of the diff
• Manual: load the unpacked extension → scan a repo twice → confirm the health sparkline shows on its library card and the History strip on its Verdict tab; a single-scan repo shows neither
• Manual: export the library → clear → re-import → confirm snapshot history survives