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/status_text.cpp

@@ -48,6 +48,11 @@ std::string format_glance(const StatusSnapshot& s) {
   out += " · ";
   out += s.window_text;
   out += " window";
+  if (s.paused) {
+    // Directly after the state word's own field rather than at the end, because it qualifies
+    // *that* word: "paused · 10 kS/s · 2 s window · for 12 s" reads as one statement.
+    out += " · for " + format_elapsed(s.paused_for_s);
+  }
   if (s.trigger_on) {
     out += " · ";
     if (s.have_event) {
@@ -74,6 +79,7 @@ std::string format_detail(const StatusSnapshot& s) {
                 s.plot1_traces, s.plot1_samples, s.buffer_mb, s.span_s,
                 static_cast<unsigned long long>(s.time_stride));
   std::string out = buf;
+  if (s.paused) out += " | paused for " + format_elapsed(s.paused_for_s);
   if (s.trigger_on) {
     out += s.have_event ? " | last event " + format_elapsed(s.since_event_s) + " ago"
                         : " | no event yet";