perf(cli): lazy-load DLP deps — 6x faster startup, optionalDependencies

[cdot65]

Summary

airs --help went from ~0.35-0.45s to ~0.06s. The DLP generate command's import chain (sharp, pdf-lib, docx + the whole embed/generate graph) loaded on every invocation; it now loads via dynamic import only when the command runs. sharp/pdf-lib/docx/piexifjs moved to optionalDependencies per user decision.

Changes

• loadGenerateCorpus() lazy loader in dlp/generate.ts with actionable missing-dep error (pnpm add sharp pdf-lib docx piexifjs), exit 1
• package.json: 4 packages → optionalDependencies
• Docs: generate.md documents optional deps

Verification

• Parked node_modules/sharp → --help works, dlp generate prints install hint
• Startup measured 3x: 0.06s real (baseline 0.33-0.45s)
• 643/643 tests, lint + tsc clean

Checklist

• TDD — failing tests committed first
• Gates green
• Docs + changeset (minor)

Closes #266