feat: Mode::Full + batch tunnel client (#94)

Adds a new `mode: full` that tunnels ALL traffic end-to-end through Apps Script → a remote tunnel node. Browser does TLS directly with the destination. No MITM, no CA installation needed on the client device.

Ships as part of the 3-PR series: #93 (tunnel-node service + CodeFull.gs, merged) + this (Rust-side Mode::Full + batch tunnel client) + #95 (Android UI dropdown, now rolled into this PR post-rebase).

### Architecture
- Client → mhrv-rs → script.google.com (Apps Script fetch) → tunnel-node on user's VPS → real destination
- Apps Script is the transport to reach the VPS; works even when the ISP blocks direct VPS IPs
- Batch multiplexer collects data from all active sessions and ships one Apps Script request per tick

### Safety properties of this merge
- AppsScript + GoogleOnly dispatch paths are **unchanged**; Full mode is an additive branch at the top of `dispatch_tunnel`.
- `tunnel_client.rs` is a new isolated module (387 LOC).
- `tunnel_request()` is a new method on `DomainFronter`, no change to `relay()` / `relay_parallel_range()`.
- Config: additive `Mode::Full` variant + validation tests (2 new); existing validation rules untouched.
- Local build: clean compile. `cargo test --quiet`: 75 passed (73 → 75 with 2 new config tests).

### Closes
Unblocks the feature requested in #61, #69, #100, #105, #110, #111, #113, #116.

### Testing
vahidlazio has iterated on prior review feedback. End-to-end testing with a real tunnel-node deployment will follow post-merge from @Feiabyte (volunteered in #61). Post-merge CI will exercise compile + full test matrix across all targets; any regression caught there gets a fast-follow fix.

tunnel-node/README.md

@@ -79,3 +79,11 @@ TUNNEL_AUTH_KEY=your-secret PORT=8080 ./target/release/tunnel-node
 ```
 
 ### Health check: `GET /health` → `ok`
+
+## Performance: deployment count and pipeline depth
+
+The mhrv-rs client runs a pipelined batch multiplexer in full mode. Each Apps Script round-trip takes ~2s, so the client fires multiple batch requests concurrently — the pipeline depth equals the number of configured script deployment IDs (clamped 2..12).
+
+More deployments = more concurrent batches hitting the tunnel-node = lower per-session latency. With 6 deployments, a new batch arrives every ~0.3s instead of every 2s.
+
+The tunnel-node itself is stateless per-request (sessions are keyed by UUID), so it handles concurrent batches naturally. For best results, deploy 3–12 Apps Script instances across separate Google accounts and list all their deployment IDs in the client config.

tunnel-node/src/main.rs

@@ -360,13 +360,6 @@ async fn handle_batch(
     (StatusCode::OK, [(header::CONTENT_TYPE, "application/json")], json)
 }
 
-fn compress_gzip(data: &[u8]) -> Vec<u8> {
-    use std::io::Write;
-    let mut encoder = flate2::write::GzEncoder::new(Vec::new(), flate2::Compression::fast());
-    let _ = encoder.write_all(data);
-    encoder.finish().unwrap_or_else(|_| data.to_vec())
-}
-
 fn decompress_gzip(data: &[u8]) -> Result<Vec<u8>, String> {
     use std::io::Read;
     let mut decoder = flate2::read::GzDecoder::new(data);