docs: close remaining spec/implementation documentation gaps

[kmturley]

Summary

Last item from an internal spec-compliance audit: five places where the spec was silent about or contradicted shipped, tested behavior.

• Added a "Computed package fields" section documenting installed, verified, and downloads — real fields the manager/registry compute and attach to package metadata, previously undocumented anywhere.
• Noted the two-tier Package Validation (structural errors vs. non-fatal recommendations) next to the Scan logic section, since the recommendations system (packageRecommendations()) was implemented but never mentioned in the spec.
• Windows App directory table now says %AppData% (matching the actual, platform-idiomatic implementation) instead of the literal $HOME — the code was right and the spec was wrong here, not the other way around.
• Install logic: reworded the Admin-privileges step to describe the conditional elevation (skip it when a compatible archive is available, added for issue Archive plugin installs unnecessarily require administrator elevation #83) and reordered it after file filtering to match actual execution order.
• Install logic: installer execution is described as "wait for it to complete" rather than "run the process in a separate thread" — fileInstall() is fully synchronous (execFileSync), so "separate thread" was inaccurate in both particulars, not just imprecise.

Docs-only change, no code touched.

Test plan

• npm run check passes (format, lint, build, tests — 207/207, unchanged since this is documentation-only)

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