docs: the readouts strip hides when empty

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

docs/claude/invariants-readouts-and-cursors.md

@@ -66,8 +66,12 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   dock cell without going through those two setters leaves the strip stale. The strip keeps the dock's
   rules: the same fixed-width ellipsized cells (who 6, type 10, value 9 characters, full text in the
   tooltip), the delta cell reads `--` on the same branches, and the cursor chips are *insensitive* with the
-  cursors off and never hidden -- through `dock_set_cursors_sensitive()`, which writes the dock's grid and the
+  cursors off -- through `dock_set_cursors_sensitive()`, which writes the dock's grid and the
   strip's chip box together; a second `gtk_widget_set_sensitive()` on either is two spellings of one state.
+  The one place the strip hides what the dock only disables: with the cursors off the strip's Cursors group is
+  not shown, with no measurement its Measures group is not, and with neither the strip is not (the dock keeps
+  both blocks, `no measurements` included; the strip has no placeholder). That is
+  `sync_readouts_strip()`, not a sensitivity.
   `[cursors-probe]` / `[measures-probe]` `strip: ... same_text=1` asserts it.
 - **`format_measure()` is the one formatter for both panes, and its SI prefix does not depend on
   there being a unit.** The cursors pane's values and the measurements pane's `text=` go through