feat(sde): bulk-load downloader + offline system/station lookups

Implements issue #51. Adds a Fuzzwork SDE downloader/importer and
expands the offline lookup layer beyond item types.

- sde.py: SDE_URL constant (FAMILIAR_SDE_URL override) and
  async download_sde() that streams + bz2-decompresses the dump
  incrementally to sde_path(), resetting the cached connection.
- sde.py: system_by_id/system_by_name/station_by_id offline lookups,
  all degrading to None when the table/file is missing.
- universe.py: get_system/get_station prefer the SDE when available,
  falling back to ESI otherwise (signatures unchanged).
- __main__.py: `familiar sde download|status|path` subcommands.
- tests: download_sde via injected MockTransport, new lookups + None
  fallbacks, and SDE-vs-ESI fast-path/fallback for get_system/station.
- README: short offline-SDE setup note.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -33,6 +33,17 @@ uv run familiar status  # list logged-in characters
 
 Re-run `login` for each character (switch EVE accounts in the browser as needed); they all live in one encrypted store. Changing the requested scopes requires logging in again.
 
+### Offline SDE (optional)
+
+For network-free item / system / station lookups, install a local copy of EVE's Static Data Export (the Fuzzwork SQLite conversion):
+
+```bash
+uv run familiar sde download   # download + decompress the SDE (a few hundred MB)
+uv run familiar sde status     # show whether it's installed and where
+```
+
+Once present, `get_item` / `get_system` / `get_station` serve from it without hitting ESI; familiar falls back to ESI whenever the SDE is absent or doesn't know the id.
+
 ### Connect to Claude
 
 Claude Code: