Centralize collection dispatch to reduce duplication

[oritwoen]

The collection name → module mapping is duplicated in 6+ places:

• `lib.rs::get()` — universal puzzle lookup
• `lib.rs::all()` — iterator over all puzzles
• `cli.rs::cmd_list()` — list dispatcher
• `cli.rs::cmd_search()` — search dispatcher
• `cli.rs::cmd_author()` — author dispatcher
• `cli.rs::cmd_export()` — export dispatcher (internally builds collection list)

Each one has its own match block with the same collection names. When a new collection is added (like arweave in #111), every location needs updating. Missing one causes silent bugs — #115 is an example where ballet was added to the module but not to CLI dispatchers.

A central registry (e.g. a `Collection` enum or a static dispatch table) that maps names to `all()`/`author()`/`get()` would let each new collection be registered once.