fix(organizer): treat categories as lazy destinations

[samtruman]

Summary

Treat the organized root as the required library location while allowing category directories to remain optional until an item is actually routed there.

What changed

• fail closed when the organized root is missing or unusable
• keep unused category directories optional
• create destination categories lazily for real organizer runs
• keep dry-run non-mutative
• preserve generic category handling without special cases

Tests

Added focused Organizer tests covering:

• missing organized root
• unused categories
• lazy generic category creation
• dry-run directory safety
• existing category behavior

Validation: targeted Organizer tests, full unit/e2e/regression suite, typecheck, backend build, and diff check.