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.

cmd/builddict/main.go

@@ -17,7 +17,7 @@ import (
 )
 
 func main() {
-	dict := flag.String("dict", "dictionaries/english/sowpods.txt", "word list file (one word per line)")
+	dict := flag.String("dict", "sources/scrabble_en/sowpods.txt", "word list file (one word per line)")
 	out := flag.String("out", "testdata", "output directory")
 	name := flag.String("name", "sowpods", "base name for the output file")
 	minLen := flag.Int("min", 2, "minimum word length")