feat(output): report scan completeness

[luojiyin1987]

Summary

Report scan completeness data in terminal, JSON, and HTML output.

• Add partial-scan diagnostics to terminal output.
• Add completeness fields to JSON output.
• Add completeness banners to single-folder and multi-folder HTML reports.
• Aggregate multi-folder diagnostics by code.

This PR excludes ratchet and baseline behavior.

Part of #907.

Validation

• npm run build
• Targeted output, CLI, and multi-folder tests: 291 passed.
• Full test run: 1,251 passed. 29 offline tests need the fixture database in this local environment.

[sonukapoor]

Hi @luojiyin1987, heads up - this one picked up a conflict with main after #927 (a small status-line fix) landed in printers.ts, which this PR also reworks heavily. Could you rebase on the latest main and resolve? Once it is clean I will review it properly alongside #937. Appreciate the work on the completeness series.

[sonukapoor]

Really nice work @luojiyin1987 - this renders the completeness model cleanly across terminal, JSON, and HTML, and the detection-vs-remediation split is exactly right: a remediation-only gap keeps the headline intact and just notes fix guidance is incomplete, while a detection gap flips it to a partial-scan warning so a zero-findings result never reads as clean. Escaping and tests are solid. Two small things before it goes in.

• src/scan/completeness.ts (the fetch message): small grammar bug that is now user-visible - pluralize(count, "fetch") prints "packument fetchs" for count > 1. Same fix we just landed for "vulnerability" in #927: pass the explicit plural, pluralize(count, "fetch", "fetches"). A quick test with count > 1 would keep it from regressing.
• src/output/html-reporter.ts (renderCompletenessBanner): you extracted getCompletenessImpact() in this same PR, but the banner here still does the two inline .some() checks - could you route those through getCompletenessImpact(completeness) so there is one source of truth?
• Non-blocking, for later: the .diag-item severity/impact CSS classes have no matching rules yet, and the JSON carries both status and complete. Leave as-is or tidy in a follow-up.
• Coordination: I would like to merge this one first (it adds the shared helper), then have #937 rebase onto it.

[sonukapoor]

The plural fix and DRY reuse of getCompletenessImpact() look good - thanks for addressing both. Verified the fix commit directly against the diff, all checks pass.

[sonukapoor]

Merged - thank you @luojiyin1987!