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/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>