fix: the t0 caption is chrome anchored to the axis, not an annotation in the data

The caption that names the relative x labels' anchor was an `Annotation`, whose anchor
is a data coordinate -- so it crawled across the plot under a pan and drifted under a
zoom, which is how it was reported. It is now `Plot::set_x_note()`: the bottom row's
caption, drawn by the renderer in the axes' bottom-left corner in pixels, on the
legend's own backplate.

The text stays the host's, because only the host knows that label + caption has to be
the exact absolute time (`%.6g`, where `format_time()` would round to three figures).

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

CLAUDE.md

@@ -1437,6 +1437,21 @@ far the largest file.
   is exactly enough), so the bare comparison picked a different form from one frame to the next.
   The guard is `[timelabels-probe]`'s `distinct=1`, which asserts what the form is *for* rather
   than which form a given window gets.
+- **The `t0 = …` caption is chrome, never an annotation.** It is `Plot::set_x_note()` (added to
+  `deps/plotter` for this), drawn by `Renderer::draw_x_note()` on the legend's own
+  `draw_readout_panel()` backplate in the bottom-left corner of the axes, in **pixels**. It was an
+  `Annotation` first, and an annotation's anchor is a *data* coordinate: the caption therefore
+  crawled across the plot under a pan and drifted under a zoom, which is what "[t0=...s] move
+  slightly around when user zoom/pan" was. Three things follow. The **text is the host's**, because
+  the caption has to name the anchor precisely enough that label + caption is the exact absolute
+  time and `format_time()` would round it to three figures (`"t0 = %.6g s"`); only the **bottom
+  row** of the figure carries one, every other row being set to `""`; and the renderer measures it
+  rather than `Plot::layout()` — it reserves no space and nothing can be picked on it, so there is
+  no rect to read back, and a caption explaining the x labels must not be able to move the layout
+  those labels were measured into. The guards are the library's
+  `the_x_note_is_chrome_that_a_pan_or_a_zoom_does_not_move` (the footprint is the pixels a frame
+  with the caption differs from one without, and a pan plus a 64× zoom must give the same box to
+  the pixel) and `[timelabels-probe]`'s `note=`, which reads `Plot::x_note()` back.
 - **`fit_x()` restores the *whole of what the view may show*, which is not always `window_s_`.**
   Live, that is the newest `window_s_` at the live edge (`set_time_window(window_s_)`, the same
   path a resume takes). Pinned — a held event, a filling capture or a paused view — it is the