initial release: Rust port of MasterHttpRelayVPN apps_script mode

Faithful port of @masterking32's MasterHttpRelayVPN. All credit for
the original idea, protocol, and Python implementation goes to him.

Implemented:
- Local HTTP proxy (CONNECT + plain HTTP)
- MITM with on-the-fly per-domain cert generation via rcgen
- CA auto-install for macOS / Linux / Windows
- Apps Script JSON relay, protocol-compatible with Code.gs
- TLS client with SNI spoofing (connect to Google IP, SNI=www.google.com,
  inner HTTP Host=script.google.com)
- Connection pooling (45s TTL, max 20 idle)
- Multi-script round-robin for higher quota
- Header filtering (strips connection-specific + brotli)
- Config-driven, JSON schema matches Python version

Deferred (TODOs in code):
- HTTP/2 multiplexing
- Request batching / coalescing / response cache
- Range-based parallel download
- SNI-rewrite tunnels for YouTube/googlevideo
- Firefox NSS cert install
- domain_fronting / google_fronting / custom_domain modes
  (mostly broken post-Cloudflare 2024, not a priority)

13 unit tests pass, 2.4MB stripped release binary.

.github/workflows/release.yml

@@ -0,0 +1,108 @@
+name: release
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+permissions:
+  contents: write
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        include:
+          - target: x86_64-unknown-linux-gnu
+            os: ubuntu-latest
+            name: mhrv-rs-linux-amd64
+          - target: aarch64-unknown-linux-gnu
+            os: ubuntu-latest
+            name: mhrv-rs-linux-arm64
+          - target: x86_64-apple-darwin
+            os: macos-latest
+            name: mhrv-rs-macos-amd64
+          - target: aarch64-apple-darwin
+            os: macos-latest
+            name: mhrv-rs-macos-arm64
+          - target: x86_64-pc-windows-gnu
+            os: windows-latest
+            name: mhrv-rs-windows-amd64
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          targets: ${{ matrix.target }}
+
+      - name: Install MinGW toolchain
+        if: matrix.target == 'x86_64-pc-windows-gnu'
+        id: msys2
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: MINGW64
+          update: true
+          install: mingw-w64-x86_64-gcc
+
+      - name: Install cross-compilation tools
+        if: matrix.target == 'aarch64-unknown-linux-gnu'
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y gcc-aarch64-linux-gnu
+          echo '[target.aarch64-unknown-linux-gnu]' >> ~/.cargo/config.toml
+          echo 'linker = "aarch64-linux-gnu-gcc"' >> ~/.cargo/config.toml
+
+      - name: Configure GNU linker
+        if: matrix.target == 'x86_64-pc-windows-gnu'
+        shell: pwsh
+        run: |
+          $gcc = "${{ steps.msys2.outputs.msys2-location }}\mingw64\bin\gcc.exe" -replace '\\','/'
+          New-Item -ItemType Directory -Force -Path $env:USERPROFILE/.cargo | Out-Null
+          Add-Content -Path $env:USERPROFILE/.cargo/config.toml -Value '[target.x86_64-pc-windows-gnu]'
+          Add-Content -Path $env:USERPROFILE/.cargo/config.toml -Value "linker = '$gcc'"
+
+      - name: Build
+        run: cargo build --release --target ${{ matrix.target }}
+
+      - name: Package (unix)
+        if: runner.os != 'Windows'
+        run: |
+          mkdir -p dist
+          cp target/${{ matrix.target }}/release/mhrv-rs dist/${{ matrix.name }}
+          chmod +x dist/${{ matrix.name }}
+
+      - name: Package (windows)
+        if: runner.os == 'Windows'
+        shell: pwsh
+        run: |
+          New-Item -ItemType Directory -Force -Path dist
+          Copy-Item target/${{ matrix.target }}/release/mhrv-rs.exe dist/${{ matrix.name }}.exe
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: ${{ matrix.name }}
+          path: dist/${{ matrix.name }}${{ runner.os == 'Windows' && '.exe' || '' }}
+
+  release:
+    needs: build
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - uses: actions/download-artifact@v4
+        with:
+          path: dist
+          merge-multiple: true
+
+      - name: Release
+        uses: softprops/action-gh-release@v2
+        with:
+          files: dist/*
+          generate_release_notes: true