Do not enlarge images on open

PLAN.md

@@ -1048,7 +1048,8 @@ dev-updater (Kotlin 2.4.x, CMP 1.11.x, JDK 21).
      system-bar regions** (2026-09-12). It hides the status and navigation bars
      independently when the fitted, zoomed or panned image reaches them, and
      restores either one when it does not. `100%` recenters at one bitmap pixel
-     per screen pixel; opening still fits the whole image first.
+     per screen pixel; opening only shrinks an image that needs it to fit and
+     never enlarges a smaller one.
    - **All transcript text is selectable, from one `SelectionContainer`
      around the whole list.** Not per row: a transcript is one body of text,
      so a selection has to run from a reply into the tool output under it —