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>

README.md

@@ -344,6 +344,43 @@ array of per-node structs was measured and bought **nothing** — at six nodes t
 L1-resident either way. It was kept because a node's state belonging on one cache line is the
 right shape for a graph that grows, not because it showed up.
 
+### What the ingest filter and a still frame cost
+
+Two costs that are paid once per *wire sample* and once per *tick* respectively, and so are the
+largest two the application has that are not proportional to anything the user asked for.
+
+**The ingest filter's group test.** Decimation at ingest is an average, never a stride, so the
+filter reads every sample by design — and the test for "is this sample the last of its group"
+was a modulo: `tick % period == period - 1`, once per channel per sample, plus once more for the
+shared clock. `Partition::time_stride` is chosen at *runtime* (`plan_partition()` doubles it from
+1 until the window fits), so that modulo is not a compile-time strength reduction but a real
+64-bit `div` — and the code's own comment claimed the opposite. Both factors are powers of two
+(every per-channel `stride[i]` is 1 by design), so the period minus one is a **mask** and the test
+is `(tick & mask) == mask`. The masks are built where the accumulators are sized, keyed on the
+partition, so a re-plan cannot leave them addressing the previous partition's groups; the
+reserved-hole path got the same treatment, because a hole's samples go through the same
+accumulators and the two phases have to agree sample for sample. The shared clock's test is now
+computed once per sample and reused by the time ring, the channel loop and the math push — it was
+three tests of the same thing.
+
+**A frozen frame.** The tick queued a GL render unconditionally, sixty times a second, for the
+whole life of the process. In `Held` and `Paused` that is the same picture again: the rows are
+bound to an immutable snapshot on a pinned axis and the producer is feeding the live ring, not the
+series on screen. On llvmpipe, or on a battery, re-drawing a frozen event at 60 Hz is the entire
+cost of the application. The gate is now two parts, and the second is what makes the first safe to
+keep coarse: a *signature* of what a frozen view can be re-framed by — a new snapshot, a pan or
+zoom (the group's range, where both the gestures and the pin write), a row added or removed —
+which renders the moment it changes, and a **floor** that renders anyway every four ticks (~15 Hz),
+so anything the signature does not name is late rather than lost. A gate that can only be opened by
+a list of state is a gate that goes wrong the first time the list is one item short. A change opens
+the gate for three frames rather than one, because the figure lays itself out per *render* and its
+left gutter is a monotone floor that takes two passes to converge. The layout editor is excluded
+outright: its buttons are placed from the rects the last frame was laid out with, so there a
+skipped frame is a stale *widget* and not just a stale picture. Interaction needs none of this —
+`PlotArea` queues its own render from the pointer, scroll and drag handlers, so a crosshair and a
+cursor drag stay immediate whatever the gate decides. `[pause-probe]`'s `skips=` field reports it,
+cumulative: 0 on the live line, non-zero on the paused one.
+
 ### Statistics
 
 **Statistics** under the measurements list adds a second line to every measurement: `n`, the