docs: help pages, probe reference and session docs updated for the three new features

- 08-trigger.txt: names all four condition shapes and the buffer search; fixes the first
  paragraph and 'How to reach it' to match what the panel actually contains (fix 3).
- 10-measurements.txt: adds a Statistics bullet — one observation per recompute, what
  resets the history, what does not (fix 3).
- probe-readings.md: re-quotes condition-probe lines with level2= and mode= fields;
  corrects leg-2 window geometry (0.2 s / 10 % pre, not 1.0 s / 50 %); moves the
  wide-window reason to legs 1 and 3; fixes the search-probe scheduling note
  (search_probe calls condition_probe directly; do not also set CONDITION_MS) (fix 6).
- verification.md: corrects shape-2 window geometry to match the code (fix 7).
- invariants-session-and-export.md, README.md: document the measure_stats session key —
  position, type, default false (fix 8).

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

src/help/08-trigger.txt

@@ -1,13 +1,16 @@
 <b>Trigger</b>
 
-The trigger watches one signal for an edge through a level and captures a window around it.
+The trigger watches one signal for a condition and captures a window around it. The
+<b>Shape</b> dropdown chooses what counts as an event; the arming, hold-off, pre/post window
+and Normal/Single mode are the same for all four shapes.
 The captured window is frozen on screen until you re-arm.
 
 <b>How to reach it</b>
 
 The trigger button in the toolbar (<tt>t</tt>) reveals the trigger panel: the signal, the level,
-the edge (rising, falling, either), the mode (Normal or Single) and the pre-trigger share.
-<b>Arm</b> starts watching; <tt>a</tt> arms and disarms from the keyboard.
+the shape, the edge (rising, falling, either), the mode (Normal or Single) and the pre-trigger
+share. <b>Arm</b> starts watching; <tt>a</tt> arms and disarms from the keyboard.
+<b>Search the buffer</b> is at the foot of the same panel.
 
 <b>Worth knowing</b>
 
@@ -27,6 +30,21 @@ the edge (rising, falling, either), the mode (Normal or Single) and the pre-trig
 • The trigger reads the <b>stored</b> signal — decimated and averaged — not the wire signal.
   A spike narrower than one decimation group is filtered away rather than triggering on data
   the buffer does not contain.
+• The four shapes: <b>edge</b> — a crossing of the level in the chosen direction, the classic
+  behaviour. <b>Pulse width</b> — a crossing and its return, qualified by how long the excursion
+  lasted: less than, greater than, within or outside a pair of widths; the event is timed at the
+  closing edge. <b>Window</b> — the signal entering or leaving a band between two levels; the
+  panel keeps the two levels at least one increment apart, because a zero-width band reduces the
+  condition to a float-equality test that never fires on a real-valued signal. <b>Dropout</b> —
+  the absence of a crossing for longer than a timeout; one event per outage, re-arms when a
+  crossing returns.
+• The shape's own rows are the only ones shown; the rest are hidden, so the panel never offers
+  a pulse width to a window trigger.
+• <b>Search the buffer</b> runs the same condition over the whole retained ring and lists every
+  place it holds. The arrows step through hits; each step pauses on the hit, centred, exactly as
+  the Pause button does. Up to 16 hits in the visible window are marked on every row; when there
+  are more the panel says how many and suggests zooming in. The search uses the trigger's own
+  condition, set in one place, so a hit is by construction an instant the trigger would have fired.
 
 <b>Example</b>
 

src/help/10-measurements.txt

@@ -45,6 +45,12 @@ set, with <b>Save</b> and <b>Delete</b>.
   small value is <tt>0.00</tt>.
 • Cost is bounded by reading the summary pyramid the producer already maintains, not the raw
   window. The values are still exact: a bucket's sum is a sum.
+• <b>Statistics</b> adds a second line to every measurement showing <tt>n</tt>, the mean, the
+  standard deviation and the range. Each observation is one <i>recompute</i> of the pane, so
+  <tt>n</tt> counts windows measured, not samples — a mean frequency over many windows is what
+  that claims to be. <b>Reset</b> clears every accumulator and starts fresh. A change of display
+  window, a CONFIG or a measurement edit also resets; a pause, a resume, a zoom, a pan and the
+  toggle itself do not, so turning the line off to read the plot and back on again keeps the history.
 
 <b>Example</b>