docs: the reseq probe's readings, and the invariants resequencing changes

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

README.md

@@ -26,7 +26,7 @@ Options: `--server`, `--port`, `--multicast`, `--interface`, `--buffer-mb`
 (size of the sample buffer, default 200), `--max-window` (largest time
 window the toolbar offers, default 600), `--holdoff` (default trigger hold-off),
 `--recv-buffer`, `--plots` (the number of stacked plots to start with, 1..8; rows are
-added and removed from the **Layout editor** afterwards).
+added and removed from the **Layout editor** afterwards), `--relative-time` (see below).
 
 Every CONFIG is dumped to stderr once -- publish mode and, per signal, its name,
 element count, rate, time mode and time-signal index -- because a signal that
@@ -363,6 +363,50 @@ latch, a pan and a zoom do **not** reset: they change which samples are in view,
 exactly what a statistic over many windows is meant to average. The toggle itself does not
 reset either, so turning the line off to read the plot and back on again keeps the history.
 
+## Resequencing and relative timing
+
+**A shaded band marks samples the stream says exist and have not arrived.** When a packet is
+received that reveals a gap in the counter, a run of NaN samples is reserved for the missing
+packets; the span is shaded in the plot and the counters in the status bar show it. The band
+carries three states:
+
+- **Solid** while the hole still lacks packets.
+- **Faint** once the late packets arrive and repair the values — the data is now correct, but
+  the record remains that this region arrived out of order.
+- **Absent** once that part of the stream scrolls out of the ring's buffer.
+
+Two things are **not** repaired when a late packet arrives: **math channels** and **the trigger**.
+A backfilled slot's value is corrected in the ring and in the pyramid, so the raw trace and the
+readouts are right. The math engine and the trigger already ran over the placeholder, however, and
+are not replayed: a math trace keeps its gap across a repaired hole, and if the trigger latched
+adjacent to one, that latch was on the data the engine actually saw.
+
+**Known limitations.** The fade is all-or-nothing at the hole level — a hole with one packet still
+missing shades solidly, even if all but one slot are already repaired. And the two tints are not
+yet configurable; the preferences dialog in `todo.md` is where that would land. Do not use a
+command-line flag for either; none exists.
+
+**`--relative-time` makes the x axis follow the sample count and the declared rate**, rather than
+the stream's own clock. The packet counter says exactly how many samples each gap spans, so a
+reserved hole is a precise arithmetic range. In stream mode the same hole is estimated from the
+declared cadence, which is the existing glitch-bridge mechanism reused.
+
+The history cost of relative mode is measured rather than computed:
+
+| mode | retained span |
+| --- | --- |
+| stream time (default) | **83.886 s** |
+| `--relative-time` | **41.943 s** |
+
+Conditions: the mock at 100 kS/s, 4 channels at stride 1, the default `--buffer-mb 200`.
+Relative mode retains **exactly half** the history — not the ~22% the original spec claimed, and
+not the 75% a per-byte calculation suggests. The reason is the floor: relative mode keeps a second
+f64 ring for absolute time beside the stream clock's, so a tick's cost goes from
+`8 + 4×4 = 24` bytes to `16 + 4×4 = 32` bytes — a third more. But `Partition::time_capacity` is
+floored to a **power of two** (`src/model.cpp:83`, `:110`), so a third more weight does not cost a
+third of the span; it costs a whole halving. The measured figures above are the correct ones; a
+per-byte computation gives the wrong answer for this reason.
+
 ## The session layout file
 
 What you arranged is remembered per data source, in