fix(tunnel-client): use portable_atomic::AtomicU64 for 32-bit MIPS

PR #153's connect_data instrumentation imported `AtomicU64` directly
from `std::sync::atomic`, which works on every release target except
the 32-bit MIPS OpenWRT target (`mipsel-unknown-linux-musl`) — that
platform has no hardware-backed 64-bit atomics, and Rust's std type
isn't even defined there. The v1.4.0 release workflow's mipsel build
failed with `error[E0432]: no AtomicU64 in sync::atomic`.

`domain_fronter.rs` already imports `portable_atomic::AtomicU64` for
the same reason — `portable-atomic` (already in Cargo.toml with the
"fallback" feature) provides a software-emulated 64-bit atomic on
targets that need it. Apply the same import here for the new
preread_* counter and connect_data_unsupported flag.

`AtomicBool` stays in std — it works on every target, no polyfill
needed.

Verified locally: cargo build + cargo test --lib (91/91 pass) clean
with the import change.

Same v1.4.0 — no version bump. Re-runs the release workflow to publish
the missing mhrv-rs-openwrt-mipsel-softfloat artifact alongside the
existing v1.4.0 release assets. Telegram notification suppressed for
this re-publish (same version, no point re-posting).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/tunnel_client.rs

@@ -6,7 +6,14 @@
 //! 30 in-flight requests — matching the per-account Apps Script limit.
 
 use std::collections::HashMap;
-use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
+// `AtomicU64` from `std::sync::atomic` requires hardware-backed 64-bit
+// atomics, which 32-bit MIPS (`mipsel-unknown-linux-musl` — our OpenWRT
+// router target) does not provide — the std type isn't even defined
+// there, so the build fails with `no AtomicU64 in sync::atomic`. We
+// already pull `portable-atomic` for `domain_fronter.rs` for the same
+// reason; reuse it here. `AtomicBool` works fine in std on every target.
+use portable_atomic::AtomicU64;
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
 use std::time::{Duration, Instant};