Separate internal WireGuard encryption from data carriers

AWG-only policy also selected a competing end-to-end plugin, so compatible
external links could still have no compatible encrypted data channel. Carrier
changes replaced tunnel state and status selected counters from different
engines. Use one internal WireGuard plugin and a separate DataCarrier contract
for external capabilities/lifecycle. Keep the logical link, keys, handshake,
interface and counter epoch across policy, fallback and direct/relay changes.
Control/discovery transport selection and membership remain independent.

Share the bounded per-neighbour UDP WireGuard engine between ordinary WG and
Amnezia masking. Encrypt the relay envelope inside the outer hop channel with
fresh pair keys/PSK. Remove duplicate WG key/device code and the TCP/TLS inner
cipher. TLS carries opaque internal WireGuard frames. Refuse missing native
transports instead of letting default iroh impersonate their protocol names.
Probe the actual authenticated carrier, independently of the shared IP tunnel;
failed local probe submissions count as loss too.

Apply the vendored iroh interface exclusion to actual direct candidates, not
just public discovery records. Exclude overlay subnets from introductions and
propagate the actual owned interface name, including kernel-assigned utun names,
to every carrier. Production logs showed paths through the overlay itself.
This removes a recursive transport path; it does not establish the historical
cause of the reported 100 GB absolute counter. Exit-node traffic is also real
peer IP traffic even when local applications are idle.

Hide inner cipher choices/handshake clutter in CLI and tray. Report external
routes, readiness and actionable policy/version failures. Count original IP
packets only when their ciphertext is accepted by the logical transport,
including queued packets' exact unpadded lengths. Exclude physical probes and
keepalives. Export counter epochs; sum per-peer rate deltas and discard stale
histories so joins/restarts cannot turn old totals into fresh traffic. Status
also prints exact IP byte totals for comparison with the tray.

Coordinated upgrade: data ALPN 6, internal wireguard-e2e capability v4,
wg/awg v2, QUIC carrier v1, TLS v2, local IPC v23. Control ALPN 8, device/network
identity derivation, signed records and saved network choices stay unchanged.
Keep the historical wg-quic key store; transactionally import missing keys from
wg then awg without replacing canonical keys. Forget removes legacy copies too,
on the blocking pool, to prevent resurrection after restart. Preserve historic
cipher options as shared settings, rejecting contradictory values.

Validation on Linux in the Nix development shell:
- cargo fmt --all -- --check
- cargo clippy --locked --workspace --all-targets -- -D warnings
- cargo test --locked --workspace --all-targets (562 passed, 3 ignored)
The inherited service socket/secret/group environment overrides were removed
for isolated CLI process tests. A new carrier test initially raced the first
peer announcement; its deadline polling now waits for a peer row explicitly.
Real loopback endpoints, membership, SQLite, WG/AWG/TLS encryption and memory
TUNs cover mixed-carrier relaying and one smoltcp TCP connection surviving
relay/direct/policy changes and a dropped transmission. Counter tests check
exact plaintext lengths, refused sends, epochs, joins and stale histories.
Public DHT, privileged kernel and manual benchmark tests remain ignored;
macOS/Windows native TUNs, desktop GUI and WAN/DPI behavior were not run.

docs/protocol.md

@@ -15,9 +15,9 @@ Upgrading the crate or bumping the control protocol must never change an
 existing `NetworkId`.
 
 Control version 8 adds signed, multi-writer VIP field snapshots to version 7's
-node retirements. Data ALPN remains `tsunagi/data/5`. All members must upgrade
+node retirements. Data ALPN is `tsunagi/data/6` for the single internal WireGuard channel. All members must upgrade
 together. Network ids, derivation labels, handshake encoding and existing record
-signatures are unchanged. Local CLI/tray IPC is version 22; restart the agent
+signatures are unchanged. Local CLI/tray IPC is version 23; restart the agent
 with matching clients. SQLite schema 10 adds `vip_fields`, preserving identity,
 network configuration, signed node records and retirement proofs.
 See [virtual services](vip.md), [state sync](sync-model.md) and [AWG](amnezia.md).
@@ -145,7 +145,7 @@ event does not report a network id.
 ## The data plane protocol
 
 IP plugin packets never travel on a control connection. They use their own
-ALPN, `tsunagi/data/5`, on their own iroh connection:
+ALPN, `tsunagi/data/6`, on their own iroh connection:
 
 ```text
 initiator -> responder : (the same membership handshake as above)
@@ -192,7 +192,7 @@ and version 5 added the end-to-end protocol to it:
 | source peer | 32 | original endpoint public key |
 | destination peer | 32 | final endpoint public key |
 | flow id | 8 | big endian opaque stable hash supplied by the plugin |
-| protocol | 8 | big endian FNV-1a of the end-to-end protocol id (`wg`, `tcp-tls`, ...) |
+| protocol | 8 | big endian FNV-1a of the internal protocol id (`wireguard-e2e`) |
 | payload | remaining | end-to-end encrypted plugin bytes |
 
 The fixed header is 82 bytes. Zero/oversized hop limits, unknown versions,