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/09-cursors.txt

@@ -37,6 +37,11 @@ owns a <b>Reset</b> button. Drag a marker to move it. The mode dropdown offers f
   is drawn larger than a trace's sample dots so it does not read as one more sample.
 • The time row exists only in the modes that show the vertical pair. <tt>horizontal</tt> has no
   time row, by design.
+• A cursor <i>stays where you put it</i> while a live view rolls under it, which is what a bench
+  scope does: its <tt>T0</tt> is an age, so a marker left at <tt>-1.5s</tt> keeps reading the
+  sample a second and a half old as the stream advances. A measurement's marks do the opposite —
+  they travel with the sample they measured — because a cursor is a question you asked and a mark
+  is an answer about one particular sample.
 
 <b>Example</b>