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.
Standalone Rust/axum HTTP server + Apps Script-side CodeFull.gs for users who want to deploy a remote tunnel node. All new files; no changes to the main Rust crate. This is part 1 of 3 of the full-tunnel feature — it adds scaffolding that users can opt into once the Rust-side Mode::Full lands in #94.
vahidlazio