Improve tray VIP monitoring and table layout

The VIP editor put routing and traffic observations in a dense text block,
while nested layouts in the devices grid could let text escape its row.
Separate the VIP overview from configuration and draw both live tables with
explicit cell bounds, font-aware row heights, clipping and horizontal overflow.
Measure columns before painting and keep widths stable as live values change.

The overview reuses the existing traffic sparklines for service totals and
individual listener backends, with TX/RX rates, health, RTT, open flows and
counters. Retain bounded histories by service/listener/backend identity and
release them when their resources disappear. Recreated listeners start fresh.

Use egui 0.31 Scene and Painter for a pannable, zoomable routing diagram with
Fit, clickable backend details and linked table selection. Highlight recent
counter growth separately from the backend selected for new connections;
idle bindings alone do not imply traffic, and stale activity expires. Draw
legend markers directly rather than depending on optional font glyphs.
The overview only reads saved observations; edits and deletion still require
Save. No dependencies, wire formats, IPC or persistent schemas change.

Validation on Linux through nix develop:
- cargo fmt --all -- --check
- cargo clippy --locked --workspace --all-targets -- -D warnings
- cargo test --locked --workspace --all-targets: 566 passed, 3 ignored
- cargo build --locked -p tsunagi-tray
The inherited TSUNAGI_CONTROL_SOCKET was unset for Clippy and tests.
Seven new GUI tests cover enlarged fonts, mixed online/offline rows, long
names, large counters, narrow/wide layouts, scene clicks, traffic selection,
stale samples, counter resets, listener recreation and history cleanup.
The first enlarged-font regression exposed an override-font height mismatch;
row sizing now resolves the actual selected font and the regression passes.
A disposable fixture preview rendered successfully with software OpenGL on
an isolated Xvfb display. Initial Xvfb attempts needed explicit xkbcommon and
OpenGL runtime paths in the Nix environment. The preview never contacted an
agent; its temporary source was removed. macOS, Windows, Wayland and live host
TUN traffic were not exercised by this UI change.

docs/vip.md

@@ -43,7 +43,23 @@ listeners, backend nodes/ports, check settings and flow timeouts. All edits,
 including deletion, remain drafts until **Save**. Status updates do not
 replace a dirty draft. A save conflict retains the draft; **Discard / reload**
 loads the latest configuration. Closing a dirty window offers explicit discard
-or continued editing. Addresses and ids are available in a collapsed detail.
+or continued editing.
+
+The **Overview** tab shows a total VIP traffic plot and one backend table per
+listener, with health, RTT, open flows, totals, TX/RX rates and individual
+traffic plots. Plots switch between bytes and packets. Observations describe
+traffic seen by this device, not a network-wide aggregate.
+
+An interactive diagram connects the service name through each listener to its
+node and target port. Drag to pan, Ctrl+scroll/pinch to zoom, and use **Fit**
+to restore the view. Click a backend in the diagram or table to highlight it
+and see its identity, address and measurement age. Green branches with moving
+dots indicate counter growth during the latest status sample; a blue ring
+marks the backend selected for new connections. Existing flows can therefore
+keep a different branch active. Idle open flows alone do not light up a branch;
+activity expires after five seconds without a fresh sample. Graph interaction
+never edits or saves configuration. The **Configuration** tab retains the
+explicit Save workflow. Addresses and ids are available in a collapsed detail.
 
 ## Delivery and recovery