Build infrastructure for releases

- Makefile: cross-compile to linux/{amd64,arm64}, darwin/{amd64,arm64},
  windows/amd64 into dist/ with SHA256SUMS. Version is stamped via
  -ldflags '-X main.version=...'.
- .gitea/workflows/release.yml: on tag push (v*) the workflow runs
  `make dist`, creates a Gitea release for the tag and uploads every
  artefact from dist/ via the Gitea API.
- main.go: `rb-search version` / --version prints the stamped version.
- README: new "Готовые бинари" section with curl/Invoke-WebRequest
  install snippets and a checksum-verify hint.

.gitignore

@@ -1,3 +1,4 @@
 /rb-search
+/dist/
 *.test
 *.out