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.

Makefile

@@ -8,7 +8,7 @@
 #   ru_erudit.dawg   — Эрудит (the Ё→Е folded + de-duped list, committed as russian/erudit.txt)
 #
 # CI builds the DAWGs as a validation gate; release artifacts are published from this output
-# (see README.md). Regenerate russian/erudit.txt from scrabble.txt with dictprep/fold_yo.py.
+# (see README.md). Regenerate russian/erudit.txt from scrabble.txt with tools/fold_yo.py.
 
 export GOPRIVATE := gitea.iliadenisov.ru/*
 
@@ -21,13 +21,13 @@ BUILDDICT := $(GO) run ./cmd/builddict
 dawg: dawg-en dawg-ru dawg-erudit
 
 dawg-en:
-	$(BUILDDICT) -dict dictionaries/english/sowpods.txt -alphabet latin -name en_sowpods -out $(DAWG_DIR)
+	$(BUILDDICT) -dict sources/scrabble_en/sowpods.txt -alphabet latin -name en_sowpods -out $(DAWG_DIR)
 
 dawg-ru:
-	$(BUILDDICT) -dict dictprep/russian/scrabble.txt -alphabet russian -name ru_scrabble -out $(DAWG_DIR)
+	$(BUILDDICT) -dict sources/scrabble_ru/scrabble.txt -alphabet russian -name ru_scrabble -out $(DAWG_DIR)
 
 dawg-erudit:
-	$(BUILDDICT) -dict dictprep/russian/erudit.txt -alphabet russian -name ru_erudit -out $(DAWG_DIR)
+	$(BUILDDICT) -dict sources/erudit_ru/erudit.txt -alphabet russian -name ru_erudit -out $(DAWG_DIR)
 
 clean-dawg:
 	rm -f $(DAWG_DIR)/*.dawg