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/erudit_ru/README.md

@@ -0,0 +1,6 @@
+# erudit_ru source
+
+`erudit.txt` — the Эрудит word list: the Ё→Е folded and de-duplicated form of
+[`../scrabble_ru/scrabble.txt`](../scrabble_ru/scrabble.txt), produced by `tools/fold_yo.py`
+(the Эрудит ruleset has no Ё tile and treats Е/Ё as one letter). Built to `dawg/ru_erudit.dawg`
+(`make dawg-erudit`).