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.

.gitea/workflows/build.yaml

@@ -34,9 +34,9 @@ jobs:
       - name: Build DAWGs
         run: |
           mkdir -p dawg
-          go run ./cmd/builddict -dict dictionaries/english/sowpods.txt -alphabet latin   -name en_sowpods  -out dawg
-          go run ./cmd/builddict -dict dictprep/russian/scrabble.txt    -alphabet russian -name ru_scrabble -out dawg
-          go run ./cmd/builddict -dict dictprep/russian/erudit.txt      -alphabet russian -name ru_erudit   -out dawg
+          go run ./cmd/builddict -dict sources/scrabble_en/sowpods.txt -alphabet latin   -name en_sowpods  -out dawg
+          go run ./cmd/builddict -dict sources/scrabble_ru/scrabble.txt    -alphabet russian -name ru_scrabble -out dawg
+          go run ./cmd/builddict -dict sources/erudit_ru/erudit.txt      -alphabet russian -name ru_erudit   -out dawg
           ls -la dawg/
           for f in en_sowpods ru_scrabble ru_erudit; do
             test -s "dawg/$f.dawg" || { echo "missing dawg/$f.dawg"; exit 1; }