Protect Linux exit-node connections and tidy the device table

Using an exit node could destroy direct peer connections: the agent's UID
bypass protected outbound routing, but reverse-path checks look up replies
without that UID and selected the overlay default. A real-host A/B check
confirmed that relaxing only strict reverse-path filtering restored direct
connections and exit HTTP traffic; the diagnostic host changes were removed.

Keep strict filtering and protect the Linux underlay with a dedicated masked
mark bit, 0x40000000. Mark the dedicated service UID in OUTPUT, save only that
bit in conntrack and restore it before PREROUTING reverse-path checks. Install
IPv4/IPv6 mark and UID main-table bypass rules before Tailscale's priorities;
leave application-specific and exit-default routing after them. Preserve
foreign mark bits, main-table routes and foreign firewall/policy rules.

Protect the first inbound TCP/UDP packet using bounded, locally owned socket
addresses/ports from /proc and an actual socket match. Refresh socket protection
before sending private transport negotiations and on discovery ticks, without
firewall writes for an unchanged socket set. Remote announcements never become
OS arguments. A failed refresh leaves active exit routing in place. Install
protection before routes, remove routes before protection, roll back partial
activation, and remove only exact interface tags, including legacy UID rules.

Reject strict kernel rp_filter without src_valid_mark with an actionable error;
never weaken filtering or change host sysctls. Document mark-aware Netfilter
requirements and provide the trusted ip6tables path in the NixOS module.
Other platforms retain their existing exit implementation. Peer wire formats,
identity derivation, local IPC and persistent storage are unchanged.

Make the tray devices grid fill and shrink with the window, constrain the first
cell and give remaining width to traffic plots. Keep displayed end-to-end
protocol and its short encryption tooltip consistent, independent of carrier
protocol. Move handshake/session age into its own column beside the existing
iroh control RTT. Use stable per-device hover identities and concise tooltips;
missing protocols/measurements remain explicit rather than invented.

Validation on Linux through nix develop:
- cargo fmt --all -- --check
- cargo clippy --locked --workspace --all-targets -- -D warnings
- cargo test --locked --workspace --all-targets: 529 passed, 3 ignored;
  inherited TSUNAGI_CONTROL_SOCKET and TSUNAGI_CONTROL_GROUP were unset
- explicit privileged kernel regression: 1 passed, using disposable namespaces,
  real TCP/UDP IPv4/IPv6, strict kernel and Netfilter RPF, first inbound packets,
  dynamic UDP binding, application exit routing/IPv6 blocking, repeated apply
  and exact cleanup with foreign Tailscale-priority rules retained
- headless egui grid/scroll-area rendering at multiple sizes and control RTT,
  mark masks/order, passive socket parsing, partial failure and cleanup tests
- git diff --check

Earlier iterations caught rtnetlink family API and nonexhaustive initializer
errors, Clippy warnings, egui's implicit minimum column width, and kernel-test
fixture issues (wildcard echo source, blocked IPv6 neighbour discovery and a
closed child stdout pipe). Corrected before final checks; the stdout-only test
fix was checked with fmt, Clippy and the explicit kernel regression afterward.
Native tray rendering and macOS/Windows checks were not run. The running host
agent was not replaced or restarted. Requested release tag: v0.1.0-rc.32.

docs/testing.md

@@ -207,4 +207,24 @@ future cutoffs, bounds, deterministic merge and resistance to stale snapshots.
 SQLite tests migrate schema 8 to 9 and inject counter-write failure to prove
 atomic rollback. The CLI test starts two real agent processes and exercises
 offline cleanup by network and hostname through the control socket.
-Tray rendering and physical screen sizing are not exercised automatically.
+Native tray rendering and physical screen sizing are not exercised automatically.
+
+Linux exit-client unit tests cover preservation of foreign mark bits, matching
+only the agent's UID and locally owned TCP/UDP sockets, exact tagged cleanup,
+partial IPv6 failure and mark-aware strict kernel RPF validation. The ignored
+`overlay::hostrules::linux_exit_kernel::strict_firewall_preserves_underlay_and_application_exit_routing`
+test creates two disposable network namespaces. It reproduces the UID-only
+failure with real sockets, then runs TCP/UDP IPv4/IPv6 and first inbound TCP/UDP
+packets, including a UDP port bound after activation, through real mark/conntrack
+rules with both strict kernel and
+Netfilter reverse-path checks. It verifies application exit routing, IPv6
+blocking, repeated apply and cleanup with foreign rules at Tailscale's priority
+retained. It requires root, iproute2,
+iptables/ip6tables and Python; it does not run in the default suite or claim
+to test the entire mesh inside these namespaces.
+
+Devices-window tests render the egui grid at multiple widths with its real
+scroll area. They check stretching and shrinking without an expanding first
+column, separate handshake/control RTT values, unknown measurements and short
+end-to-end hints that do not substitute the carrier protocol. This does not
+establish native desktop rendering on every platform.