Add reference data mappings and auto-download for PokeDB import tool

Add mappings module with pokemon form, location area, encounter method,
and version mappings. Auto-download PokeDB JSON exports from CDN on
first run, caching in .pokedb_cache/.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -60,8 +60,9 @@ temp/
 *.tmp
 *.temp
 
-# PokeAPI fetch cache
+# PokeAPI / PokeDB data cache
 .pokeapi_cache/
+.pokedb_cache/
 
 # Go build output
 tools/fetch-pokeapi/fetch-pokeapi