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>

src/channel.cpp

@@ -78,6 +78,18 @@ void ChannelBuffer::invalidate() noexcept {
   pending_amends_.clear();
 }
 
+void ChannelBuffer::republish_from(u64 absolute) noexcept {
+  published_ = absolute;
+  window_x0_ = 0.0;
+  pending_amends_.clear();
+  // `clear()` leaves the pyramid empty at absolute 0, and `drop_before()` past the end
+  // restarts an empty window at the index it is given -- which is the only way to say
+  // "nothing is published, and the next append is sample `absolute`". The no-op guard is
+  // for `absolute == 0`, where `clear()` has already said exactly that.
+  pyramid_.clear();
+  if (absolute > 0) pyramid_.drop_before(absolute);
+}
+
 f64 ChannelBuffer::time_of(u64 absolute_index) const noexcept {
   // Channel sample `j` is produced on the tick that also pushes time slot
   // `j*stride + stride - 1` (see `Acquisition::on_data`): the producer stamps the