fix: a clock that stalled on a re-plan, and a dialog that could not close

**The clock.** Trigger mode showed a blank plot and never latched when the session defined a math
channel, because such a session gives the model a plan *before* the stream's own signals are in it:
one derived channel, declared rate 0, so `plan_partition()`'s 1 kHz fallback is the interval those
early frames are stamped at -- 1300x too fast for a 1.3 MS/s stream -- and they have nowhere to go,
since the value loop skips derived channels. Relative mode then stamped a *position* times the plan's
*current* interval, so when the CONFIG's channels landed and the plan took the real rate (769 ns) the
same positions produced times 1300x *smaller* than what was already in the ring, and the monotonic
clamp held the clock at the old value for the rest of the run: one repeated stamp, `history=0`, no
latch, and a window the renderer cannot draw. The same family as the restarted-client poisoning
fixed in 620473a, through a different door. The filter's own output was finite and correct
throughout, and disabling the math channel only appeared to cure it because that is a structural
change which rebuilds the store -- the third time that channel has looked guilty while being a
bystander.

Relative mode now accumulates *seconds* (`rel_base_s_ += n * dt`), so a re-plan is a slope change and
the clock cannot jump backwards by construction; a frame is only stamped into a model that has a
*streamed* channel; and the epoch is adopted once per connection, with "unset" a state rather than
the value zero. A one-shot warning names the family if it returns: `the clock stalled at ...: N
samples stamped [a..b] are all behind it`.

**The dialog.** The Configurations window's Close button and the window-manager close both did
nothing: `close-request` was connected to a bare `void` lambda, and that signal is `gboolean` -- TRUE
means "handled" and suppresses the default handler that destroys the window, while `G_CALLBACK` is a
cast, so GTK read whatever the compiler left in the return register. A standalone GTK program of that
shape reproduces it: `void` handler -> `window_alive=1` after `gtk_window_close()`, `gboolean FALSE`
-> 0. The same audit found `gtk_window_present(GTK_WINDOW(...))` on an `AdwDialog`, a type assertion
and a no-op; it is `adw_dialog_present()` now. The menu row stays: it is the only way to switch or
save a configuration without restarting, and what made it look useless was that it could not close.

Verified: trigger mode with that session's graph reads `ring=[0.0058..0.2446] -> [0.0058..1.9969]`,
`history 0.237 -> 2.0 s`, `edge at 1.120 s, latched at 2.120 s`, `display=event(frozen) latches=1`,
`traceless paints 266` against 1519 before the fix. Baselines hold: 0 errors, the documented three
warnings, test-ui 13 ok, test-help green, `gtk warnings: 0`.

docs/claude/invariants-trigger-and-pinning.md

@@ -197,6 +197,24 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   `process()` uses that for `last_trigger_s_`, `pending_start_s_` **and** the pre-window gate —
   all three, or the marker, the cut and the gate would disagree with each other by the width of
   a pulse.
+- **The trigger's whole input is the stored clock, so a clock that stops is a trigger that never
+  fires — and *that* is what a blank trigger-mode display is.** `Acquisition` feeds
+  `TriggerEngine::process(value, rel)` with the same `rel` it stamps the ring with, and the engine
+  has no other clock: the pre-window gate measures the event against the retained history
+  (`event_t - oldest_s_ >= pre_seconds_`), the post window counts *stream seconds* past the edge,
+  and `holdoff_until_s_` is set from `now_s` at the arm. A timeline that stands still therefore
+  reads as `history=0.0000/1.0000 s` for ever, on every line of `trig engine:`, no matter how many
+  samples arrive — and with no latch, `display_mode()` never leaves `Waiting`, whose bind is
+  *nothing* by design, so the plot is blank and the other traces are blank with it (the renderer's
+  bound window is empty too, so the same emptiness shows with the trigger off). Nothing about that
+  picture says "clock": it is "in trigger mode, with a math channel enabled, the data disappears",
+  which is how it reached a user. The reading is the `trig engine:` line's own pair — **`n` climbing
+  while `ring=[a..a]` stands still** — and the one-shot
+  `[udpscope] warning: the clock stalled ...` that `on_data()` prints at the first whole frame whose
+  stamps are all behind the clock. See
+  [invariants-data-path.md](invariants-data-path.md) for the two entries (a plan whose interval
+  moved under the stream; a frame accepted into a model with no streamed channel) and for why the
+  relative clock is accumulated in seconds.
 
 ## A latch is also a file