feat(transport,cli,tunnel): v3.4 port auto-detect + bug fixes from live test

Live macOS test against the production server uncovered six bugs (one of which
turned out to be a port collision with sing-box, not a real bug); this commit
addresses all of them and adds v3.4 port discovery so the same collision is
handled transparently next time.

## v3.4 server port-discovery

- Defaults moved off 443/444 to 8443/8443/8444 (TransportSection::default,
  ServerInitOpts, ProvisionClientOpts, CLI flags). 443 is heavily contested in
  practice (sing-box, Hysteria2, reverse proxies) and the previous default
  silently lost the bind when a co-tenant was already there.
- MultiServer::bind_with_outer_or_scan: scans forward up to
  DEFAULT_PORT_SCAN_MAX (20) candidates per transport when the requested port
  is occupied; QUIC keeps walking if it lands on the custom-UDP port.
- MultiServer::bound_addrs(): the actual addresses each transport bound to.
- Server logs the bound addresses and writes a runtime snapshot
  (server.toml.runtime.json) when they differ from the requested ones, so
  `aura sign-bridges` can re-sign the bridges manifest later.
- BridgeManifest gains an optional `endpoints: Vec<BridgeEndpoint>` field
  with per-transport ports. Backward-compatible: old v3.3 clients ignore the
  field and continue to use the v1 `bridges` line.
- `aura sign-bridges --endpoints HOST:tcp=N:quic=N:udp=N` to mint v3.4
  manifests; bridges line is auto-synthesised for v3.3 clients.

## Bug fixes from the live test

- macOS TUN naming (#41): the tun crate rejects names that don't match
  ^utun[0-9]+$. On macOS we now substitute `""` (kernel auto-assigns utunN),
  capture the assigned name via inner.tun_name(), and propagate it through to
  os_routes::OsRouteGuard::install — so `route add -interface utunN` uses
  the real interface, not "aura0".
- Packet counters (#42): Stats { tx_packets, rx_packets } are now actually
  bumped by the data path. `aura status` shows live numbers instead of
  permanent zeros.
- render_client_toml schema (#44): provisioner emits proper
  `[[tunnel.split.vpn]] cidr = "..."` / `[[tunnel.split.direct]]` blocks from
  new --vpn-cidrs / --direct-cidrs flags. The v3.3 `vpn_cidrs = [...]` flat
  array was silently ignored by serde, leaving users with `rules: 0` even
  when their CIDRs looked right.
- #43 / #46 (TCP/443 dial early-eof / no payload back): diagnosed as the
  sing-box port collision, not an Aura bug. The v3.4 port-scan path makes it
  go away — the server picks a free port and clients learn it from the
  manifest.

## Test coverage

Three new unit tests for the port-scanner (UDP busy, TCP busy, zero budget);
two new tests for v3.4 BridgeManifest round-trip with endpoints; one
integration test for the new `[[tunnel.split.vpn]]` rendering; tests for the
runtime-state file write/read round-trip; agent-added router-counter tests
in aura-tunnel/tests/routes.rs.

cargo test --workspace, cargo clippy --workspace -- -D warnings, and
cargo fmt --check all pass.

#45 (silent client exit when underlying QUIC transport breaks) is still
outstanding — needs deeper investigation; deferred to a follow-up.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/aura-transport/src/lib.rs

@@ -72,7 +72,9 @@ pub mod tcp;
 pub mod udp;
 
 pub use conn::AuraConnection;
-pub use dial::{dial, Accepted, DialConfig, Endpoints, MultiServer, TransportMode};
+pub use dial::{
+    dial, Accepted, DialConfig, Endpoints, MultiServer, TransportMode, DEFAULT_PORT_SCAN_MAX,
+};
 pub use mimicry::{alpn_protocols, chrome_quic_transport_config, ALPN_H3, DEFAULT_SNI};
 pub use padding::{
     inject_padding_frames, next_bucket_for_profile, pad_to_bucket, pad_to_https_size,