feat: the x axis is relative to now, and an export carries both clocks

Three asked-for changes, one commit because they are one coordinate system.

The live axis is a fixed [-window, 0] with the newest sample at 0, so x reads
as an age and the labels stand still while the trace rolls under them. That
makes the display clock distinct from the stream clock for the first time, so
`ScopeModel::stream_at_x0()` becomes the single conversion between them
(`display_x = stream_t - stream_at_x0()` in all four modes) and a snapshot
carries its own origin in `EventSnapshot::x_offset`. The pause view is now
rebased like an event's, reversing an earlier deliberate choice: a frozen
absolute-stamped picture would sit a whole uptime right of a newest-relative
axis. That also fixed a latent bug, the export adding `trigger_time()` to a
*paused* view's stamps.

The status bar says how long you have been paused -- wall time, since the
producer never stops, through `format_elapsed()` so the field cannot widen.

The export writes two time columns: `time`, the axis as displayed, and
`abs_seconds`, the stream's own clock for the same sample. f64 seconds and
not the wire's integer, because `udps_frame_t::values` is a `const double*`
and the integer never exists in this process.

Cursors and measurement marks move in opposite directions under the rolling
axis, deliberately: a cursor is a position on screen and stays put, a mark
names a sample and travels with it. `App::measure_window` is stream-clocked
for a sharper reason -- a display-coordinate live window never moves, so the
"the window moved" recompute would never fire again.

Moves the deps/plotter pin to 58473db, which stops a time label's mantissa
reaching its own radix: a one-second tick differenced off a live axis read
`1e+3ms`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/help/06-navigate.txt

@@ -24,6 +24,11 @@ The full table is in the shortcut overview (<tt>Ctrl+/</tt>).
   zoom, so a 2 s window zoomed into reads 2 s again. The window is more than the axis: it is also
   the trigger's event span and how the samples are stored, so a wheel notch that redefined it
   would re-plan the buffer and drop your history.
+• The axis does not scroll — the <i>data</i> does. A live axis is a fixed <tt>[-window .. 0]</tt>
+  with the newest sample at 0, so the labels stand still and the trace rolls under them. The
+  cursors stand still with the labels (they are positions on screen, so a cursor you placed stays
+  where you put it, over newer data each frame), while a measurement's marks travel with the
+  sample they measured and leave the view when it does.
 • A live view is clamped to the window you asked for, not to what the ring happens to hold.
   That is why a buffer that has only just started filling leaves its left-hand side empty
   instead of squeezing the window down to the first few samples.