perf: a frozen frame is not re-rendered sixty times a second

The tick queued a GL render unconditionally. In `Held` and `Paused` the rows
are bound to an immutable snapshot on a pinned axis, so a frame in which none
of the picture's own terms moved is the same frame again. The gate keys on a
coarse signature of what a frozen view can be re-framed by, opens for
`kSettleRenders` frames on a change because the figure lays out per render, and
has a floor that renders anyway every few ticks. The layout editor is excluded:
its buttons are placed from the last frame's rects, so a skipped frame there is
a stale widget rather than a stale picture. `skips=` in the probe dump is the
reading.

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

docs/claude/verification.md

@@ -148,7 +148,7 @@ precedent, not an exception.
 - `UDPSCOPE_PROBE_PAUSE_MS` — pause acquisition mid-run so layout numbers are
   measured on a still picture. It now also asserts what a pause *is*
   (`[pause-probe] <action> paused=D ring_produced=N event_size=M frozen_x=[..] axis=[..]
-  glance=[..]` on stderr):
+  skips=N glance=[..]` on stderr):
   it presses the **button**, not `Acquisition::set_paused()`, because pausing is a display
   state as well as an acquisition one and the view snapshot and axis pin live in the
   handler. Across the three lines **`ring_produced` must grow** (the producer never
@@ -168,6 +168,17 @@ precedent, not an exception.
   stops while paused and "how long have I been looking at this" is a question about the user, not
   about the stream; it is written through `format_elapsed()` so the field steps to a coarser unit
   instead of growing digits (see the layout rule).
+  **`skips=` is the assertion for the render gate** (the end of `tick()`), cumulative over the
+  session: the `live` line must read **0** and the `paused` line must read **more than 0**. A
+  frozen view is bound to an immutable snapshot on a pinned axis, so a frame whose picture cannot
+  have changed is not drawn again — and this is the only line that can say whether that happened,
+  since a skipped frame is invisible in every other reading by construction. Non-zero on the
+  `live` line is the gate's condition inverted, which would cost dropped samples' worth of
+  picture; 0 on the `paused` line is the gate not engaging and the saving not being made.
+  The gate cannot stall the picture (`kStaticRenderEvery` renders every few ticks regardless),
+  so the renderer's own baselines below are unchanged by it: a measured run read `skips=0`,
+  then 33, then 45, with `columns == graded`, `render blank frames: 0`, and the layout floor
+  still converged (`y-gutter 104..104`).
 - `UDPSCOPE_PROBE_COLOR_MS` — recolour one plotted signal and read the colour back out of the
   trace and the chip, round-trip it through the session writer, then clear it. Schedule
   `UDPSCOPE_ENABLE_ALL_MS` before it: with nothing bound it has no trace to recolour and asserts