Stage 14: solver & dictionary split — consume published module + DAWG artifact (TODO-1/TODO-2)

- backend/go.mod pins gitea.iliadenisov.ru/developer/scrabble-solver v1.0.0; the engine's
  imports use the published module path; go.work drops the solver replace (GOPRIVATE fetches
  it directly from Gitea). The solver's wordlist/dictdawg are now public packages.
- CI (go-unit, integration): drop the solver sibling-clone, set GOPRIVATE, and download the
  dictionary DAWG release artifact (scrabble-dawg-<DICT_VERSION>.tar.gz from the new
  scrabble-dictionary repo) for BACKEND_DICT_DIR.
- Docs: ARCHITECTURE §5/§11/§13/§14 + backend/README updated to the published-module +
  release-artifact model. PLAN.md re-scoped Stage 14 to the split and added Stages 15 (deploy
  infra & test contour), 16 (prod contour), 17 (dual Telegram bots); TODO-1/TODO-2 marked done.

backend/README.md

@@ -154,8 +154,11 @@ internal/connector/  # backend gRPC client to the Telegram connector (operator b
 
 ```sh
 docker run -d --name scrabble-pg -e POSTGRES_PASSWORD=dev -p 5432:5432 postgres:17-alpine
+# DAWGs: extract the dictionary release artifact (or point at a local scrabble-solver/dawg):
+mkdir -p /tmp/dawg && curl -fsSL https://gitea.iliadenisov.ru/developer/scrabble-dictionary/releases/download/v1.0.0/scrabble-dawg-v1.0.0.tar.gz | tar xz -C /tmp/dawg
 BACKEND_POSTGRES_DSN='postgres://postgres:dev@localhost:5432/postgres?search_path=backend&sslmode=disable' \
-  BACKEND_DICT_DIR=../../scrabble-solver/dawg \
+  BACKEND_DICT_DIR=/tmp/dawg \
+  GOPRIVATE='gitea.iliadenisov.ru/*' \
   go run ./cmd/backend
 ```
 
@@ -178,19 +181,18 @@ go run ./cmd/jetgen     # rewrites internal/postgres/jet against a temp containe
 
 ## Engine & dictionaries
 
-`internal/engine` consumes the sibling `scrabble-solver` module in-process. Its
-bare module path (`scrabble-solver`, not a URL) cannot be fetched via VCS, so the
-workspace `go.work` carries `replace scrabble-solver => ../scrabble-solver` and
-the build must run from the repository root (the workspace), not from this module
-in isolation. `github.com/iliadenisov/dafsa` (the DAWG loader) is a direct
-dependency. CI clones the public solver repository into `../scrabble-solver`
-before building (see `.gitea/workflows/`); locally, check it out next to this
-repository. Committed dictionaries (`en_sowpods.dawg`, `ru_scrabble.dawg`,
-`ru_erudit.dawg`) live in the solver's `dawg/` directory; the engine loads them
-by `(variant, dict_version)` from a directory path. Since Stage 3 the backend
-loads them at startup from the **required** `BACKEND_DICT_DIR` (a missing
-dictionary aborts the boot); the future versioned-artifact direction is recorded
-in [`../PLAN.md`](../PLAN.md) TODO-2.
+`internal/engine` consumes `scrabble-solver` in-process as a **published, versioned
+module** (`gitea.iliadenisov.ru/developer/scrabble-solver`, pinned in `go.mod`). Set
+`GOPRIVATE=gitea.iliadenisov.ru/*` so go fetches it directly from this Gitea (skipping
+the public proxy/checksum DB); no sibling checkout or `go.work` replace is needed (for
+local solver co-development you may add a temporary replace — see `go.work`).
+`github.com/iliadenisov/dafsa` (the DAWG loader) is a direct dependency. The dictionaries
+(`en_sowpods.dawg`, `ru_scrabble.dawg`, `ru_erudit.dawg`) ship as a **release artifact**
+from the [`scrabble-dictionary`](https://gitea.iliadenisov.ru/developer/scrabble-dictionary)
+repo (one semver per set); the engine loads them by `(variant, dict_version)` from
+`BACKEND_DICT_DIR`. Since Stage 3 the backend loads them at startup as a hard dependency
+(a missing dictionary aborts the boot). See [`../PLAN.md`](../PLAN.md) Stage 14
+(TODO-1/TODO-2).
 
 ## Tests
 
@@ -201,6 +203,8 @@ go test -tags=integration -count=1 -p=1 ./... # Postgres-backed (needs Docker)
 
 Integration tests are guarded by the `integration` build tag and run against a
 throwaway `postgres:17-alpine` container; they fail loudly when Docker is absent
-rather than skipping. The `internal/engine` tests load the committed DAWGs from
-`BACKEND_DICT_DIR` (defaulting to the sibling `../scrabble-solver/dawg`) and fail
-loudly when that directory is absent.
+rather than skipping. The `internal/engine` tests load the DAWGs from
+`BACKEND_DICT_DIR` (CI sets it to the extracted dictionary release artifact; locally it
+defaults to a `scrabble-solver/dawg` sibling checkout) and fail loudly when that directory
+is absent. `GOPRIVATE=gitea.iliadenisov.ru/*` is needed for go to fetch the pinned solver
+module.