fipstop: add "Listening on fips0" panel to Node tab

Surfaces local services reachable from the mesh, paired with their
current `inet fips` baseline filter classification. Lands to the
right of the existing TUN section in the Traffic block.

A new daemon control query `show_listening_sockets` returns IPv6
listeners bound to either `::` (wildcard) or the node's fd00::/8
address, each classified as Accept / Drop / Unknown / NoFirewall
against the running inbound chain. fipstop renders the result as a
table beside the Traffic counters: Accept rows in default White,
Drop / Unknown in DarkGray, a yellow banner above the table when
`fips-firewall.service` is inactive, and a trailing `*` on
wildcard binds to remind the operator the bind is not
fips0-specific.

Daemon side:

- `src/control/listening.rs` walks `/proc/net/tcp6` and
  `/proc/net/udp6` via the procfs crate (LISTEN state for TCP,
  wildcard remote for UDP), filters to fips0-reachable binds, and
  resolves inodes to PID / comm via `/proc/<pid>/fd`.

- `src/control/firewall_state.rs` shells out to
  `nft -j list table inet fips` and walks the inbound chain.
  Recognises canonical accepts (`tcp/udp dport N accept`,
  `dport { ... } accept`, `dport A-B accept`), the iifname-scoping
  line, conntrack and icmpv6 lines (skipped). Any rule with
  unrecognised matchers (saddr filters, jumps, daddr filters) or
  non-terminal verdicts forces Unknown classification for the
  ports it references. Eleven unit tests cover the classification
  logic; the listening enumerator carries a /proc-parsing test of
  its own.

- `show_listening_sockets` emits
  `{fips0_addr, firewall_active, sockets[]}` with per-row
  `{proto, local_addr, port, pid, process, filter, wildcard_bind}`.

fipstop side:

- `src/bin/fipstop/ui/dashboard.rs` splits the Traffic block into
  a 50/50 horizontal layout; the existing TUN + Forwarded panel
  occupies the left half.

- `src/bin/fipstop/ui/listening.rs` renders the right half.

- `main.rs` fetches the new query each tick when the Node tab is
  active. Errors are non-fatal: an old daemon without the query
  leaves the payload at None and the panel renders "loading...".

`Cargo.toml` gains `procfs = "0.18"` on the Linux target. IPv4
listeners are not enumerated — fips0 is IPv6-only.

Folded in: revert the default-socket lookup from writability-probe
back to existence-based selection. The previous tempfile-probe on
`/run/fips` silently steered fipstop / fipsctl onto an XDG path
the daemon never bound for any user in the `fips` group whose
shell session had not yet picked up the supplementary group (no
re-login after `usermod -aG`). `XDG_RUNTIME_DIR` is set on every
modern systemd-managed user session, so this hit the common case.
The kernel checks actual group membership at `connect(2)`, so a
user who genuinely cannot connect now gets a clear `EACCES`
rather than a silent path mismatch. Drops the now-unused
`is_writable_dir` helper. `XDG_RUNTIME_DIR` existence validation
is preserved.

Documentation:

- `docs/reference/cli-fipstop.md` — Node-tab row updated, new
  "Listening on fips0 panel" section.
- `docs/reference/control-socket.md` — `show_listening_sockets`
  added to the read-only queries table.
- `docs/how-to/enable-mesh-firewall.md` — new "Verify with
  fipstop" section.
- `docs/tutorials/host-a-service.md` — fipstop callouts at
  Steps 3, 5, 6 + Troubleshooting bullet + wildcard-bind reminder
  under "What you've learned".
- `CHANGELOG.md` — new bullet under `Added / Operator Tooling`,
  resolver `Fixed` entry rewritten to describe the
  existence-based final shape.

docs/reference/cli-fipstop.md

@@ -36,7 +36,7 @@ query on its first activation and on every refresh tick while active.
 
 | Tab | Query | Shows |
 | --- | ----- | ----- |
-| **Node** | `show_status` | Identity, version, uptime, peer/link/session counts, sparklines for mesh size, tree depth, peer count, bytes, loss. |
+| **Node** | `show_status` (+ `show_listening_sockets`) | Identity, version, uptime, peer/link/session counts, sparklines for mesh size, tree depth, peer count, bytes, loss. The Traffic block on this tab is split: TUN counters on the left, the **Listening on fips0** panel on the right (see below). |
 | **Peers** | `show_peers` (+ `show_links`, `show_transports` cross-refs) | Authenticated peers in a table. Selecting a row and pressing Enter opens a detail view. |
 | **Transports** | `show_transports` (+ `show_links`, `show_peers` cross-refs) | Tree of transport instances with per-link children when expanded. |
 | **Sessions** | `show_sessions` | End-to-end FSP sessions. |
@@ -52,6 +52,38 @@ Tree → Filters → Performance → Routing → Graphs → Gateway. The Links
 and Cache tabs are not in the cycle but are fetched as cross-references
 to populate Peers, Transports, and Routing detail views.
 
+## Listening on fips0 panel (Node tab)
+
+The right half of the Node tab's Traffic block lists local IPv6
+listening sockets reachable from `fips0`, paired with the current
+`inet fips` baseline filter classification for each (proto, port).
+The panel exists to remind the operator which local services are
+exposed to the mesh and which of those are admitted by the
+default-deny firewall.
+
+| Column | Meaning |
+| ------ | ------- |
+| **Proto** | `tcp` or `udp`. IPv4 listeners are not enumerated; `fips0` is IPv6-only. |
+| **Port** | Listening port number. |
+| **Process** | `comm(pid)` resolved by walking `/proc/<pid>/fd/`. A trailing `*` marks wildcard binds (`local_addr == ::`) — the bind is not fips0-specific, so the operator sees that the service is exposed across every interface, not just the mesh. |
+| **State** | `OPEN` (default White) — the baseline filter has a canonical accept rule for this (proto, port). `filt` (DarkGray) — chain falls through to `counter drop`. `filt?` (DarkGray) — a rule references the port but uses matchers (saddr filter, jump, daddr) the panel cannot fully decompose; operator should `nft list table inet fips` to confirm. |
+
+When `fips-firewall.service` is **not** active, the `inet fips`
+table is absent. The panel renders every row in default White and
+replaces the title with a yellow banner reading
+"`Listening on fips0  fips-firewall.service inactive — all listeners exposed`".
+
+The panel is read-only and unselectable. It refreshes on the same
+poll tick as the rest of the Node tab. Sockets owned by other users
+that the daemon could not resolve to a PID render as `?` in the
+Process column; this only happens if the daemon itself is running
+without root privileges (an unusual dev setup), since walking
+`/proc/<pid>/fd/` for processes the daemon does not own requires
+elevated capabilities.
+
+The panel is Linux-only; on non-Linux daemons the query returns an
+empty list and the panel hides.
+
 ## Keybindings
 
 ### Global

docs/reference/control-socket.md

@@ -113,6 +113,7 @@ table below lists every command currently registered.
 | `show_transports` | — | `transports[]` — `transport_id`, `type`, `state`, `mtu`, `name`, `local_addr`, optional `tor_mode`, `onion_address`, `tor_monitoring`, `stats`. |
 | `show_routing` | — | `coord_cache_entries`, `identity_cache_entries`, `pending_lookups[]`, `pending_tun_destinations`, `pending_tun_packets`, `recent_requests`, `retries[]`, `forwarding`, `discovery`, `error_signals`, `congestion`. |
 | `show_identity_cache` | — | `entries[]`, `count`, `max_entries`. Each entry: `node_addr`, `npub`, `display_name`, `ipv6_addr`, `last_seen_ms`, `age_ms`. |
+| `show_listening_sockets` | — | `fips0_addr`, `firewall_active` (bool — `inet fips` table loaded), `sockets[]`. Each entry: `proto` (`tcp` / `udp`), `local_addr` (`::` or the node's fd00::/8 address), `port`, `pid` (nullable), `process` (nullable), `wildcard_bind` (bool — `local_addr == ::`), `filter` (`accept` / `drop` / `unknown` / `no_firewall`). Linux-only; returns an empty `sockets[]` on other platforms. |
 | `show_stats_list` | — | `metrics[]` (each with `name`, `unit`, `scope`), `fast_ring_seconds`, `slow_ring_minutes`, `peer_retention_seconds`. |
 | `show_stats_history` | `metric` (req), `peer` (req for per-peer metrics), `window` (`<N>s` / `<N>m` / `<N>h`, default `10m`), `granularity` (`1s` / `1m`, default `1s`) | A single `Series`: `metric`, `unit`, `granularity_seconds`, `values[]`. |
 | `show_stats_all_history` | `peer` (optional npub), `window`, `granularity` | `granularity_seconds`, `window_seconds`, `peer`, `series[]` (one per metric). |