ci: pin Go 1.27.3 for the differential compatibility check

Go 1.25 ServeMux redirects unclean paths with 301, while newer Go (and the
Rust port that mirrors it) uses 307, so the Go reference built in CI
disagreed with Rust on '/a/../app.js'.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -9,7 +9,10 @@ env:
   CARGO_TERM_COLOR: always
   # Pin the toolchain to the declared MSRV so CI matches production.
   RUST_TOOLCHAIN: 1.90.0
-  GO_VERSION: "1.25.0"
+  # The differential check compares Rust against net/http behaviour, which
+  # changes between Go releases (e.g. ServeMux redirects 301 -> 307), so the
+  # reference must be built with the same Go the Rust port mirrors.
+  GO_VERSION: "1.27.3"
 
 jobs:
   rust:

README.md

@@ -124,7 +124,8 @@ against the same `BALANCE_DB_PATH`; do not run both writers simultaneously.
 Existing accounts, access/refresh tokens and saved device cursors remain usable.
 See [deployment and rollback](docs/DEPLOYMENT.md) for the full procedure.
 
-Verify the implementations locally with Go 1.25+, Rust and Python 3:
+Verify the implementations locally with Go 1.27.3 (the version CI uses for the
+differential check), Rust and Python 3:
 
 ```sh
 cargo test --locked

docs/DEVELOPMENT.md

@@ -2,7 +2,7 @@
 
 ## Требования
 
-Production-сервер использует Rust 1.90+ и bundled SQLite. Go 1.25+ нужен для сохранённой эталонной реализации и differential-проверок. Python 3 используется без сторонних пакетов.
+Production-сервер использует Rust 1.90+ и bundled SQLite. Go 1.25+ нужен для сохранённой эталонной реализации; differential-проверки (`tools/compatibility.py`) требуют Go 1.27.3, как в CI: поведение `net/http` (например, код редиректа ServeMux 301/307) зависит от версии Go. Python 3 используется без сторонних пакетов.
 
 Проверить локальную версию: