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/01-overview.txt

@@ -6,9 +6,15 @@ Acquisition runs on its own thread and writes a ring buffer sized from <tt>--buf
 the drawing thread reads whatever that ring holds when a frame starts, which is why a trace
 never makes the window stutter.
 
-The stream is the clock. Every time you read on screen is a timestamp the source sent, never
-your machine's, so a stalled source freezes the view instead of scrolling it -- and a source
-whose clock steps still gets the window you asked for.
+The stream is the clock. Every interval you read on screen is measured between timestamps the
+source sent, never your machine's, so a stalled source freezes the view instead of scrolling it
+-- and a source whose clock steps still gets the window you asked for.
+
+The time axis is <i>relative to now</i>. While streaming it is a fixed <tt>[-window .. 0]</tt>
+with the newest sample at 0, so x reads as an age: <tt>-1.5s</tt> is a sample a second and a half
+old. In trigger mode 0 is the event's edge instead, and a paused view keeps the ages it was
+frozen with. The source's own clock is still there when you need it -- an export writes it as a
+second column.
 
 <b>How to reach it</b>
 

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.

src/help/07-pause.txt

@@ -20,6 +20,8 @@ live ring and puts the window back on the newest data.
   zoom out is clamped to the snapshot's data.
 • Pausing is the easiest way to measure a layout or read a cursor precisely, because nothing
   under the pointer moves between frames.
+• The status bar says how long you have been paused — wall time, since the stream itself never
+  stopped. It is there so a frozen picture cannot be mistaken for a live one that went quiet.
 
 <b>Example</b>
 

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>
 

src/help/12-export.txt

@@ -17,15 +17,17 @@ the format follows the path's extension.
 • The stored rate is not always the wire rate. A window long enough to need decimation is stored
   averaged at a lower rate, and the export carries <i>that</i> — read the status bar's ⓘ popover
   for the figure before trusting a sample count.
-• Two time columns. <tt>time</tt> is the display timeline (relative to the trigger edge while an
-  event is held) and <tt>stream_time</tt> is the source's own clock. Both are strictly increasing
-  and evenly spaced; duplicate or backwards stamps would mean the file held two clocks.
+• Two time columns. <tt>time</tt> is the display timeline — negative ages ending at 0 while
+  streaming, offsets from the trigger edge while an event is held — so it is the x you read a
+  value at on screen. <tt>abs_seconds</tt> is the source's own clock for the same sample, which
+  is what lets two exports taken in different modes line up. Both are strictly increasing and
+  evenly spaced; duplicate or backwards stamps would mean the file held two clocks.
 • An export taken <i>mid-capture</i> writes the partly filled window that is on screen, which is
   shorter than the event span. Export outside a capture if you want the whole event.
 
 <b>Example</b>
 
 A 2 s window at 10 kS/s over 3 traces writes 20001 rows and 5 columns
-(<tt>time</tt>, <tt>stream_time</tt>, three signals). The <tt>.npy</tt> is
+(<tt>time</tt>, <tt>abs_seconds</tt>, three signals). The <tt>.npy</tt> is
 <tt>64·⌈(10+len(header)+1)/64⌉ + rows·(16 + 4·cols)</tt> bytes. Read it back with
 <tt>pd.DataFrame(np.load(path))</tt>.

src/help/14-status.txt

@@ -12,8 +12,10 @@ The bar is always visible under the plot. Click <b>ⓘ</b> for the detail popove
 • The badge is one glyph in three colours: lit and dim alternating means data is arriving, and a
   steady idle colour means it is not. A badge that never blinks is not a working badge.
 • The state field is one word from a fixed set: <tt>live</tt>, <tt>paused</tt>, <tt>armed</tt>,
-  <tt>capturing</tt>, <tt>event held</tt>, <tt>disarmed</tt>. In trigger mode the bar adds
-  exactly one more field, how long ago the last event was.
+  <tt>capturing</tt>, <tt>event held</tt>, <tt>disarmed</tt>. Two states add one field each: in
+  trigger mode, how long ago the last event was; while paused, how long you have been paused.
+• The paused figure is <i>wall</i> time, not stream time — the producer never stops while paused,
+  so it answers "how long have I been looking at this picture".
 • Nothing on the bar may change width with the stream, because the bar sits directly above the
   plot and a wider label would relayout it. That is why the rate collapses to one figure or a
   range rather than listing per channel, and why an elapsed time steps to a coarser unit instead