fix: keep an absent DriftScore null instead of 0

[vibgrate-team]

Summary

An unscored scan used to report DriftScore 0 and low risk — the same result as a fully current tree. A missing score now stays absent:

• JSON (vg scan --format json and .vibgrate/scan_result.json) stores "score": null and null component fields. riskLevel is none, not low.
• The text and Markdown summaries print n/a for the overall score, the risk level, and any component that was not measured.
• A score that was actually computed is unchanged, including a real 0 (current runtime) and a runtime lag of 4 or more (health 0, reported as drift 100).
• --drift-budget and a configured drift budget skip the comparison when the score is absent and do not fail the scan. The CLI says the score is absent. A measured score over budget still fails.

Fixes #160

Commands exercised

Before the change, computeDriftScore([]) returned score: 0 and riskLevel: "low", and each component was drift 0 because a missing health value was filled in as 100 and inverted.

After the change:

• pnpm dev scan /tmp/absent-scan --format json --no-graph --no-local-artifacts --offline --quiet on an empty directory prints "score": null, null components, "measured": [], and "riskLevel": "none". The progress line is Computing drift score · absent.
• pnpm dev scan /tmp/empty-pkg --format text --no-graph --no-local-artifacts --offline --drift-budget 0 on a package.json with no runtime and no dependencies prints DriftScore: n/a and Runtime/Frameworks/Dependencies/EOL Risk: n/a, exits 0, and writes DriftScore is absent; --drift-budget was not applied.

Checklist

• pnpm test passes (4906 tests)
• pnpm lint is clean (no new warnings)
• pnpm typecheck is clean
• Docs updated (README / DOCS / ARCHITECTURE) where behavior changed
• Determinism preserved — identical input still produces identical graph.json / report output (content-hashed IDs, stable sorts; no time, randomness, or filesystem-order dependence)
• No proprietary or internal references — public, Apache-2.0 content only
• Commits use Conventional Commits and are signed off (git commit -s, DCO)

Notes for reviewers

Weights and the health-to-drift inversion are unchanged for components that were measured. Only the absent path changed: null stays null instead of being stored as drift 0.