fix: a full reorder ring keeps the arriving packet and counts the overflow, instead of dropping it silently

A packet that needed a slot and found none was neither held nor delivered: no
counter recorded it and the consumer reserved a hole nothing could ever backfill.
It now commits the gap to free a slot and takes the ordering decision once more
against the sequence that commit left behind, bounded to one retry; past that it
is delivered in order with the counters it skipped as its `lost`, never flagged
late, because a packet ahead of the sequence has no hole to be looked up by. A
duplicate can now replace its slot at a full ring, as hold_packet's own comment
already promised, and the 20 ms default window is a named constant.

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

docs/claude/build-and-run.md

@@ -63,7 +63,7 @@ handler clears either pointer without knowing which dialog it is. Presenting eit
 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:
+is intentional. Five 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.
@@ -78,11 +78,22 @@ is intentional. Four things were changed:
    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`),
+   defaulted to `UDPS_REORDER_WINDOW_S_DEFAULT` (0.020) in `udps_client_config_init()`,
    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`.
+5. **The full ring never drops the arriving packet, and says so in a counter.** A packet that
+   needs a slot and finds none commits the current gap (which frees at least one, since
+   `commit_gap()` always delivers) and the ordering decision is taken once more against the
+   sequence that commit left behind — a bounded retry, because the commit can advance
+   `next_counter` past the arriving counter and turn an ahead-of-sequence packet into a behind-
+   of-sequence one. Past the retry it is delivered *in order*, with the counters it skipped as
+   its `lost`, and never flagged `late`: a packet ahead of the sequence has no reserved hole and
+   `Acquisition::backfill_from()` silently ignores a late packet it cannot place, so flagging it
+   would be the same silent loss by another route. `stats.reorder_overflows` counts the event,
+   so an undersized ring is a reading rather than a mystery; it is surfaced in the exit dump and
+   the `[reseq-probe]` line as `overflows=`.
 
 `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