Tidy sources into sources/<variant>/ + tools/

Consolidate the scattered build inputs (dictionaries/english/, dictprep/russian/)
into one sources/ tree keyed by the variant labels (scrabble_en/scrabble_ru/
erudit_ru), and move the Russian prep pipeline to tools/. The dawg outputs and
their filenames are unchanged — rebuilt byte-identical (en_sowpods/ru_scrabble/
ru_erudit) — so the release artifact and the backend are unaffected.

ru_stage2.py OUT_DIR and the ruwords flag defaults are repointed to
sources/scrabble_ru/; Makefile / CI / cmd/builddict default / README updated;
pipeline intermediates git-ignored. Verified: make dawg byte-identical to the
committed baseline, py_compile + go vet of the moved tools. The full Russian
regeneration pipeline (pymorphy3/libmorph/orfo PDF) was not run here.

sources/scrabble_ru/README.md

@@ -0,0 +1,9 @@
+# scrabble_ru source
+
+`scrabble.txt` — Russian Scrabble common nouns (nominative singular), produced by the prep
+pipeline under [`../../tools/`](../../tools/README.md) from the Russian academic orthographic
+dictionary, cross-checked against OpenCorpora and libmorph. `manual_confirm.txt` holds the
+hand-reviewed additions the pipeline merges in. Built to `dawg/ru_scrabble.dawg` (`make dawg-ru`).
+
+The pipeline's uncommitted intermediates (`orfo_dict_2025.txt`, `all.txt`, debug dumps) are
+regenerated here locally and are git-ignored.