Add fipstop TUI monitoring tool with smoothed metrics and quality indices

fipstop: ratatui-based TUI for real-time monitoring of a running FIPS daemon.

Tabs and navigation:
- 8 navigable tabs: Node, Peers, Transports, Sessions, Tree, Filters,
  Performance, Routing
- Tab/BackTab navigation with group separators in tab bar
- Table views with selectable rows, detail drill-down panels, and scrollbars

Node tab:
- Runtime info: pid, exe path, uptime, control socket path, TUN adapter name
- Identity: npub, node_addr, ipv6 address
- State summary with peer/session/link/transport/connection counts
- TUN IPv6 traffic and forwarded transit traffic counters

Peers tab:
- Table with Name, Address, Conn, Depth, SRTT, Loss, LQI, Pkts Tx/Rx
- Detail panel: identity, connection info, transport cross-reference,
  tree/bloom state, link stats, MMP metrics with LQI

Sessions tab:
- Table with Name, Remote Addr, State, Role, SRTT, Loss, SQI, Path MTU,
  Last Activity
- Detail panel: identity, session info, traffic stats, MMP metrics with SQI

Transports tab:
- Hierarchical tree view: expandable transport parents with nested links
  (▼/▶ indicators, ├─/└─ tree chars, Space/Arrow to expand/collapse)
- Transport detail: type-specific stats (UDP/TCP/Ethernet)
- Link detail: peer cross-reference with MMP metrics and LQI

Performance tab:
- Link-layer MMP: SRTT, loss, ETX, LQI, goodput per peer
- Session-layer MMP: SRTT, loss, ETX, SQI, path MTU per session
- Trend indicators (rising/falling/stable) with context-aware coloring

Routing tab:
- Routing state: cache sizes, pending lookups, recent requests
- Coordinate cache: entries, fill ratio, TTL, expiry, avg age
- Statistics: forwarding, discovery request/response, error signal counters

Tree tab:
- Spanning tree position with 16 announce stats (inbound/outbound/cumulative)

Filters tab:
- Bloom filter announce stats, per-peer fill ratio and estimated node count

MMP metrics enhancements:
- Add etx_trend DualEwma for smoothed ETX tracking
- Add smoothed_loss() and smoothed_etx() accessors (long-term EWMA)
- LQI (Link Quality Index) = smoothed_etx * (1 + srtt_ms / 100)
- SQI (Session Quality Index) = same formula for session layer
- All loss/ETX displays prefer smoothed values with raw fallback

Control socket:
- Add smoothed_loss, smoothed_etx, lqi/sqi to show_peers, show_sessions,
  and show_mmp JSON responses
- Rename fips_address to ipv6_addr in show_status and show_peers
- Add tun_name and control_socket to show_status
- FHS-compliant 3-tier default path: $XDG_RUNTIME_DIR, /run/fips, /tmp

Node extensions:
- Add started_at/uptime() to Node
- Add tun_name() accessor

Docker sidecar updates:
- TCP transport support via FIPS_PEER_TRANSPORT env var
- Build scripts include fipstop binary

docs/design/fips-configuration.md

@@ -54,7 +54,7 @@ peers:       # Static peer list
 | Parameter | Type | Default | Description |
 |-----------|------|---------|-------------|
 | `node.control.enabled` | bool | `true` | Enable the Unix domain control socket |
-| `node.control.socket_path` | string | *(auto)* | Socket file path. Default: `$XDG_RUNTIME_DIR/fips/control.sock` if `XDG_RUNTIME_DIR` is set, otherwise `/tmp/fips-control.sock` |
+| `node.control.socket_path` | string | *(auto)* | Socket file path. Default: `$XDG_RUNTIME_DIR/fips/control.sock`, then `/run/fips/control.sock` (if root), then `/tmp/fips-control.sock` |
 
 The control socket provides read-only access to node state via the
 `fipsctl` command-line tool. See the project
@@ -480,7 +480,7 @@ node:
     owd_window_size: 32
   control:
     enabled: true
-    socket_path: null                # null = auto ($XDG_RUNTIME_DIR/fips/control.sock or /tmp/fips-control.sock)
+    socket_path: null                # null = auto ($XDG_RUNTIME_DIR → /run/fips → /tmp fallback)
   buffers:
     packet_channel: 1024
     tun_channel: 1024