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

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

docs/claude/build-and-run.md

@@ -62,6 +62,28 @@ handler keyed by a pointer-to-pointer (`gpointer slot` cast to `AdwDialog**`), s
 handler clears either pointer without knowing which dialog it is. Presenting either on this
 app's plain `GtkApplicationWindow` is supported and warning-free; it was checked.
 
+`deps/udpsclient` is a vendored C99 source pair and its diff from the upstream source
+is intentional. Four things were changed:
+
+1. **A reorder ring of reassembled payloads.** When a packet arrives with a higher counter
+   than expected, the raw datagram is held in a fixed-size ring rather than decoded immediately.
+   A deadline (`now + reorder_window_s`, default 0.020 s) is set for the gap; the deadline is
+   served by a third `clamp_deadline()` call on the `select()` budget
+   (`udps_client.c:1145-1152`) so a quiet stream does not wait for the next arrival.
+2. **Late delivery replacing the outright discard.** A packet that arrives past the window — or
+   when the ring is drained by the deadline — is delivered to the callback flagged `late` rather
+   than discarded with `stats.stale_packets++; return 0`. That flag is what makes backfill
+   possible at all; without it the samples are gone before udpscope can act.
+3. **The third `clamp_deadline()`.** The existing pair handles the keepalive and the silence
+   timeout; the reorder deadline is a third call to the same helper, which already understood
+   the pattern.
+4. **`reorder_window_s = 0` as the switch.** `udps_client_config_t::reorder_window_s`,
+   defaulted to `0.020` in `udps_client_default_config()` (`deps/udpsclient/udps_client.c:1190`),
+   is the only knob. Setting it to zero disables resequencing and restores the previous
+   discard-on-late behaviour exactly — so a reported regression is bisectable without reverting
+   the source. **There is no command-line flag for the reorder window.** It is not surfaced on
+   udpscope's command line; do not document a `--reorder-window-ms`.
+
 `deps/plotter` is upstream's own repository and this app's figure support was added
 there rather than worked around here: `Figure::set_min_y_gutter()` (a floor under the
 common left gutter), `Figure::set_right_gutter()` / `Plot::set_right_gutter()` (an empty