fix: a still frame's measurements are the frame's, not the viewport's

A measurement is a statistic of the window the pane is *about*, and on a
pinned view -- a held event, a paused stream, an opened file -- that is the
frame, not the part of it on screen. The pane followed the viewport
instead, so a wheel on a latched event moved `average` and `top`: the
reported "when measuring a still frame, moving the zoom makes measurements
such as average or top change". Nothing about the data had changed; the
user was only looking closer.

The window is `RowSet::pinned_range()` while the pin frames the view and
the viewport otherwise, and the choice is made in `update_measures()` --
the call site -- rather than inside `MeasureWindowReader`, because it is
*booked* as well as read: that window is what the "it moved" test compares
against `App::measure_window`, so a gesture inside the pin spends no
recompute either, and a reader that chose its own would leave a pinned
view recomputing every gesture frame for a number that cannot change.
`pinned_range()` is now the one expression of the pin's extent, shared
with the framing (`hold_pinned_window()`, `fit_x()`), and the reader is
handed the window instead of reading the plot. Live is unchanged: there
the window really moves and the statistic follows it, and the
accumulator's rule about what resets the history is about neither.

`UDPSCOPE_PROBE_MEASUREPIN_MS` is the guard, three legs of before/after a
gesture on the row's own axis. On a latched event zoomed to a quarter
(`view=[-1..1]` -> `[-0.25..0.25]`, `pin` unchanged): `same=4/4
for_same=1 pass_delta=0`, every value bit-identical -- `average=0.000000
top=0.957975 peak to peak=2.000000 amplitude=1.915950` both sides. The
same on a pause taken from it. Since the key `for=` follows `pin=` now,
`[measure-trace]` gained the field, so a held run reads `for==pin` with
`swin` free to sit inside it. On a live view a zoom to an eighth reads
`same=0/4 for_same=0 pass_delta=3 span_after=0.125000 follows=1` with the
values moving -- the case the rule must not swallow.

Before the fix the same probe reads `same=1/4 for_same=0 pass_delta=1
span_after=0.500000` on the held event (`top` `0.957975` -> `0.957966`)
and `same=0/4 span_after=0.125000` on the pause (`top` `0.957944` ->
`0.358614`). Live reads the same either way.

Also fixed, found by the pause leg: pausing while an event is *held*
froze an **empty** picture. `on_pause_toggled()` read `stream_at_x0()`
after setting the pause state, and that accessor follows the display --
`paused` with no pause view published yet answers `-display_epoch()`,
which on a held event is 0, so the cut searched the ring at edge-relative
x, found nothing, and the frame bound an empty snapshot: every
measurement `--`, every ruler gone (`ev=650001` on the probe's line now,
`ev=0` before). The offset and the window are read first, which is the
offset the axis is actually displaying. The live case is unaffected, which
is why the pause probe, which pauses a live view, never saw it.

Docs corrected where they asserted the old half-truth: the invariants now
separate the accumulator's rule (unchanged) from the window a value is
computed over (this change), the trigger/pinning file names the pin as
load-bearing for both the framing and the numbers and records the pause
ordering rule, `verification.md`/`probe-readings.md` describe the new
probe and `pin=`, and the README and the manual say what a still frame
measures.

Checked: `cmake --build build --clean-first -j4` 0 errors and the
documented three warnings; `make -C build test-ui` 13 ok; `make -C build
test-help` green; an exit-dump run 0 Gtk-WARNING/Gtk-CRITICAL and `gtk
warnings: 0`; `[measurepin-probe]`, `[pan-probe]` (`held=1`,
`allowed=1`, `limited=1`), `[pause-probe]` (`event_size` non-zero exactly
while paused, `axis` inside `frozen_x`, `skips` 0 live / 30 paused),
`[stats-probe]` (`dn=1` across the pause), `[zoom-probe]` (`gap_px=0.0`
at every step) and `[measure-probe]` + the dump's `measures
window:` (all-types run `marks_in_window=4/4`, `pin=off` live) on the fast
mock. `measure cost:` is unchanged in kind: `raw=37298 (0.0%)` of
`covered=121704537`, 1.04 ms a pass -- the runs that read higher are the
probe's own zoomed-to-a-slice windows, where the window is legitimately
shorter than a bucket.

help/C/org.udpscope.app/measurements.page

@@ -5,7 +5,8 @@
   <info>
     <link type="guide" xref="index#analysis"/>
     <link type="seealso" xref="cursors"/>
-    <desc>A measurement is a statistic of the visible window of one signal, with optional
+    <desc>A measurement is a statistic of one signal over the window the pane is about — the
+    visible range, or the frozen frame while a view is pinned — with optional
     on-plot marks and a per-measurement statistics line.</desc>
     <credit type="author copyright">
       <name>Martino Ferrari</name>
@@ -14,10 +15,15 @@
 
   <title>Measurements</title>
 
-  <p>A measurement is a statistic of the <em>visible window</em> of one signal, recomputed as
+  <p>A measurement is a statistic of one signal over the window the pane is about, recomputed as
   that window moves. The pane lists them with their values; each can also draw its result on
   the plot.</p>
 
+  <p>That window is the <em>visible range</em> — so on a live view the numbers follow what you
+  pan and zoom to. On a <em>still frame</em> — a held event, a paused stream, an opened file — it
+  is the frame's own extent instead, so panning or zooming inside it changes nothing: the data
+  being measured has not changed and you are only looking closer.</p>
+
   <section id="reach">
     <title>How to reach it</title>
     <steps>
@@ -122,7 +128,9 @@
       a mean frequency over many windows is what that claims to be. <gui>Reset</gui> 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.</p></item>
+      not, so turning the line off to read the plot and back on again keeps the history. On a
+      still frame a pan or a zoom changes nothing for the history to record in the first place —
+      the values do not move there at all.</p></item>
     </list>
   </section>