Add Gitea Actions CI/CD workflows

- .gitea/workflows/ci.yml: run tests on every push/PR
- .gitea/workflows/release.yml: build Linux binaries on tag push
  - x86_64 (musl static)
  - aarch64 / RPi 64-bit (musl static)
  - armv7 / RPi 32-bit (musl static)
  - Auto-creates Gitea release with all artifacts
- scripts/build-macos-release.sh: build macOS binary locally and
  upload to an existing Gitea release

Release flow:
  git tag v0.1.0 && git push origin v0.1.0
  # CI builds Linux + RPi, creates release
  # Then on Mac: ./scripts/build-macos-release.sh --upload v0.1.0

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -0,0 +1,29 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    container:
+      image: rust:1.86-slim
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Run tests
+        run: cargo test -- --test-threads=1
+
+      - name: Build release
+        run: cargo build --release
+
+      - name: Check binary
+        run: |
+          ls -lh target/release/btest
+          target/release/btest --version