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.hpp

@@ -48,6 +48,16 @@ class ChannelBuffer final : public plot::data::Series {
   void acquire() noexcept;
   /// Consumer thread: forgets the published window (after a rebind or a reset).
   void invalidate() noexcept;
+  /// Consumer thread: drops the published window and re-publishes it from absolute sample
+  /// `absolute` on the next `acquire()`.
+  ///
+  /// For a ring whose contents were *rewritten in place* rather than appended to -- a math
+  /// channel recomputed over its inputs' retained window. `acquire()` is incremental, so
+  /// a rewrite that leaves `produced()` where it was publishes nothing and the pyramid goes
+  /// on describing the samples the edit replaced. `invalidate()` would do it too, but it
+  /// restarts at 0 and `acquire()` would then walk every absolute index ever produced; this
+  /// restarts at the oldest index the ring still holds, so the rebuild is one window.
+  void republish_from(u64 absolute) noexcept;
 
   /// Producer thread, under `ScopeModel::mutex()`: records that absolute sample `absolute`
   /// has been backfilled, for `acquire()` to fold into the pyramid.