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>

README.md

@@ -374,6 +374,7 @@ per-signal gain and offset:
 ```toml
 window_s = 2
 plots = 1
+measure_stats = false
 
 [[plot]]
 signals = ["Ch1", "", "", ""]
@@ -384,7 +385,8 @@ gain = 2
 offset = 0.5
 ```
 
-It is written (400 ms after the last change, and flushed on close) whenever the
+It also stores `measure_stats` (the measurements statistics toggle, a boolean, absent means
+`false`). It is written (400 ms after the last change, and flushed on close) whenever the
 window, the layout, a drop or a calibration changes, and applied once, when the
 first CONFIG names the signals -- signal *names* are the identity, because a
 CONFIG renumbers and an enable/disable rebuilds the channel list. Signals the
@@ -685,7 +687,8 @@ modes are unchanged, and only the question "did an event happen at this sample"
   because that is the first sample at which the width is known. `<` a width shorter than the
   signal's own pulses is the glitch trigger: nothing but a runt satisfies it.
 - **window** — the signal entering or leaving a band between two levels. A band is two levels,
-  so the panel refuses a zero-width one rather than quietly degrading to an edge.
+  so the panel refuses a zero-width one: it reduces the condition to a float-equality test,
+  which on a real-valued signal never fires.
 - **dropout** — the *absence* of a crossing for longer than a timeout. One event per outage:
   the condition re-arms only when a crossing returns.