docs(v0.1-product): document e-ink scroll cadence in screen layout

Pulls the scattered refresh-strategy facts (1-in-20 ghost-flush, defer to
idle ≥ 1 s, full refresh on Ctrl-S/Ctrl-G) into one user-facing paragraph
so future readers know what scroll actually looks like on this panel and
the periodic ~1 s flash is documented intent, not a bug to chase.

docs/v0.1-mvp-product.md

@@ -134,6 +134,17 @@ the visible area during normal typing; the edit area scrolls one line at a
 time. There is no "page" — just a continuously advancing strip. Embrace it,
 don't fight it.
 
+**What scroll looks like on e-ink:** every line-scroll is a partial refresh
+of the full edit area — every line moves, there is no small dirty rect for
+scroll — so each scroll costs ~200–300 ms. Per-keystroke partial refreshes
+accumulate ghosting; the render module clears it by upgrading every 20th
+refresh to a full refresh, deferred to the next idle pause ≥ 1 s so it
+usually lands during a thinking moment rather than mid-burst (implementation:
+[v0.1 technical → render](v0.1-mvp-technical.md#render--dirty-rect-to-epd)).
+`Ctrl-S` and `Ctrl-G` also force a full refresh. Net user experience:
+smooth-ish line scrolling while typing, occasional ~1 s flash on a pause or
+a save.
+
 ## Error UX
 
 | Failure                                               | What the user sees                                                       |