fix: a recompute that renumbered its ring, and readouts frozen under a zoom

Three defects, all of them a window describing data that is not there.

A math edit refilled the derived ring with `clear()`, which renumbers it
from 0, leaving `produced()` short by the inputs' retained base. Since
`live_window_locked()` clamps against every channel's `produced()`, that
one lagging channel answered `n = 0` for the live view, every pause view
and every trigger capture until a whole ring of fresh data had gone by --
the reported freeze, invisible until the ring first wraps, hence its
intermittence. It now restarts with `clear_to(base)`, and
`ChannelBuffer::republish_from(base)` puts the published window back:
`acquire()` is incremental, so a rewrite in place published nothing and
the pyramid went on describing the samples the edit replaced.

The trigger's channel is an index, re-resolved by name after a renumbering
-- but only in `rebuild_enabled_locked()`, so a math add or delete went
through `set_math_channels()` with no restore, the index was clamped to
-1, and `process()` returned at once while the panel still named the
channel. The restore moves into `rebuild_channels()`, which *is* the
renumbering, so no caller can forget it.

And `update_measures()` suppressed its "the window moved" test while an
event was held -- the one mode where a gesture is the only thing that can
move it. Zooming a latched event left the pane describing the whole
captured event. A pinned window needs no exemption: it lands on the range
it already measured and fails the 2 % test by itself.

The checks that missed all three: `UDPSCOPE_PROBE_MATHWRAP_MS` edits a
graph whose ring has already wrapped, which `math_probe` structurally
cannot reach; `[measure-trace]` now prints the mode, the window on screen
and the window the numbers describe; and the dump's `marks_in_window`
counts against the window on screen instead of against the one the marks
were computed over, where it was a tautology.

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

README.md

@@ -388,6 +388,13 @@ tick). The trigger is 2 % of the span, so the numbers track the window within a
 while the cost stays bounded by the window's motion rather than by the frame rate; one pass
 copies the visible samples of each *channel* once, however many measurements read it.
 
+"The window moved" is asked in **every** mode, including a held trigger event — which is the
+one mode where a zoom or a pan is the only thing that can move it. Held views were once
+exempted, on the grounds that a pinned window must not recompute every frame, and the result
+was a pane that kept describing the whole captured event while you zoomed into a slice of it.
+A pinned window needs no exemption: it lands on exactly the range it already measured and
+fails the 2 % test by itself.
+
 **Show on plot** marks the result on the row that plots the signal: a ruler at the level
 (`top`/`bottom`), both rulers at the extremes (`min`/`max`), the pulse's or the cycle's two
 edge times as vertical markers. The marks are **readouts, not instruments**: a fixed pool of