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>

docs/claude/build-and-run.md

@@ -209,6 +209,18 @@ adapter `PlotArea`'s **figure mode** —
 `set_figure()`, one `input::Controller` per row, nearest-row event routing, a
 figure-wide crosshair — plus `PlotArea::row_at()` for naming the row a pointer is over.
 
+`plot::data::RingBuffer` carries two writes a plain SPSC ring has no use for and this app
+does: `amend(absolute, v)` rewrites a retained slot when a late packet's data finally
+arrives, and `clear_to(absolute)` empties a ring and **resumes its count at a given
+origin**. The second exists because a math channel's ring is recomputed from its inputs'
+*retained* window: `clear()` would restart it at 0 and leave every absolute index —
+`base()`, `produced()`, `copy_from()` — short by that window's base, which is the
+desynchronisation `live_window_locked()` then reads as an empty window (see
+[invariants-math-channels.md](invariants-math-channels.md)). Its guards are
+`ring_clear_to_resumes_the_absolute_count` — a derived ring refilled from a source's retained
+window must come back with the source's own `produced()`/`base()` — and
+`ring_clear_to_zero_is_clear`.
+
 The render-quality work went there for the same reason: `kTextFS` now **filters** the
 glyph atlas (a 2×2 bilinear clamped to the instance's own cell) instead of point-sampling
 it, and `LineProgram::set_snap()` / `u_snap` rounds an axis-aligned segment's ink onto