docs: non-warhammer system drop policy (Middle-Earth default-dropped)

docs/SORTING.md

@@ -140,6 +140,14 @@ already-present duplicates, 0 copied):
   truncated/HTML dead-link downloads are listed in
   `<extracted>/.broken/RETRY_URLS.txt` (all 0807.st; re-run
   `python3 dlp.py <channel> <out> --urls-file ...` for them).
+- **Non-warhammer systems are dropped** (2026-10-07, user decision:
+  "drop anything that isn't really warhammer"). The sorter default
+  (`DROPPED_SYSTEMS` in `dlphelper/sorter.py`) is `{Middle-Earth}`:
+  catalog entries of dropped systems are filtered at load and keyword
+  hits for them count as unclassified, so nothing is ever placed into
+  those sections - the files stay in `_unsorted`. `--drop-system NAME`
+  drops more, `--keep-system NAME` re-enables one. The committed catalog
+  snapshot itself is untouched.
 
 ## Catalog file (`data/library_catalog.json`)