Structural validation still diverges from mcs pack validate #14

The heuristic and file-reference layers match mcs pack validate (verified against mcs 2026.9.27 on every registered pack). The structural layer, validateTechpackYaml in src/lib/validator.ts, still disagrees with mcs ExternalPackManifest.validate() / the loader on some verdicts. All of these predate #13.

Registry says invalid, mcs accepts (false invalids — fix first)

• Component description: "" — registry requires a non-empty string; mcs has no such check.
• Schema maxLength / non-empty limits enforced as errors.
• Component-level scope: validated.

mcs rejects, registry says active

• Duplicate copy destination (ManifestError.duplicateDestination).
• Required fields on doctor checks.
• Dotted template sectionIdentifier (dotInRawID).
• installAction decode failures (e.g. mcpServer without name, shorthand without destination).
• minMCSVersion that doesn't parse or is newer than the current mcs.
• Non-integer hookTimeout (e.g. 1.5).

Diff ExternalPackManifest.swift at the release tag against validateTechpackYaml, check by check, and add a test per case.