docs: a strikethrough that renders, and the 22% claim attributed to the right document

SPECS.md

@@ -39,13 +39,15 @@
 
 ## Deviations from the spec
 
-The following were built differently from what the spec states, for documented reasons in `README.md`:
+The following were built differently from what was asked for, each for a reason `README.md` states
+in full:
 
 - **`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.
+  of two, so a third more weight costs a full halving. The design document and the implementation
+  plan both put the cost at ~22%, which is wrong; the per-byte figure of 75% is 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.