[BUG] org-habit-build-graph breaks on recent master with unicode glyphs [9.8-pre (9.8-pre-526a7d)]

[General discussions about Org-mode.]

When using Unicode glyphs, org-habit-build-graph breaks due to a recent
change in Emacs master. Emacs master disallows resizing unibyte strings
to multibyte strings.

(setq org-habit-overdue-glyph ?•
      org-habit-alert-glyph ?•
      org-habit-today-glyph ?•
      org-habit-completed-glyph ?◦)

Instead of mutating strings, a better approach in org-habit-build-graph
could be to concatenate a new string? It seems the goal of upstream
Emacs is to move away from string mutations? I added Mattias Engdegård
in cc.

Thanks.

Emacs  : GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.49, 
cairo version 1.18.4)
 of 2025-08-25
Package: Org mode version 9.8-pre (9.8-pre-526a7d)