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

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

SPECS.md

@@ -36,3 +36,16 @@
   - the time should be computed from the time associated with the signal (not from the client but from the streamed data)
   - in trigger mode data visualisation should not be refreshed till next event, that's why 2 identical buffer, one is the currently
     viewed event, the other used to acquire live data for next event
+
+## Deviations from the spec
+
+The following were built differently from what the spec states, for documented reasons in `README.md`:
+
+- **`todo.md` item 2 — relative timing.** Implemented as `--relative-time`. The retained history
+  in this mode is **41.943 s** against the default **83.886 s** (measured at 100 kS/s, 4 channels,
+  `--buffer-mb 200`): exactly half, because a second f64 ring for absolute time raises a tick's
+  cost from 24 to 32 bytes — a third more — and `Partition::time_capacity` is floored to a power
+  of two, so a third more weight costs a full halving. The spec and the original plan both quoted
+  ~22%, which is wrong; the per-byte figure of 75% is also wrong for the same reason.
+  Absolute stream time is preserved in a separate ring and used for the export's `abs_seconds`
+  column, exactly as `todo.md` requires.