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.

Makefile

@@ -0,0 +1,33 @@
+VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
+LDFLAGS := -s -w -X main.version=$(VERSION)
+DIST    := dist
+
+PLATFORMS := \
+	linux/amd64 \
+	linux/arm64 \
+	darwin/amd64 \
+	darwin/arm64 \
+	windows/amd64
+
+.PHONY: build dist clean
+
+build:
+	CGO_ENABLED=0 go build -ldflags '$(LDFLAGS)' -o rb-search .
+
+dist:
+	@rm -rf $(DIST)
+	@mkdir -p $(DIST)
+	@for plat in $(PLATFORMS); do \
+		os=$${plat%/*}; arch=$${plat#*/}; \
+		ext=""; if [ "$$os" = "windows" ]; then ext=".exe"; fi; \
+		name=rb-search-$(VERSION)-$$os-$$arch$$ext; \
+		echo "  build $$name"; \
+		CGO_ENABLED=0 GOOS=$$os GOARCH=$$arch \
+			go build -trimpath -ldflags '$(LDFLAGS)' -o $(DIST)/$$name . || exit 1; \
+	done
+	@cd $(DIST) && sha256sum rb-search-* > SHA256SUMS
+	@echo "built artefacts:"
+	@ls -lh $(DIST)
+
+clean:
+	rm -rf $(DIST) rb-search