Windows: herdr icon codepoints (U+E1A0…) render as U+FFFD in panes and sidebar, but render correctly in Windows Terminal outside herdr #5201

Environment

• herdr 0.9.3 (stable) and preview build 2026-10-12-6d697e1e75ff, x86_64-pc-windows-msvc
• Windows 11 (build 26300)
• Windows Terminal, font GeistMono NFM (Nerd Font Mono)
• Icon font Herdr Agent Icons Max installed for the current user (per-user registration). It covers U+E1A0–U+E1BA and U+E1C0–U+E1C5.

What happens

Herdr's private-use icon codepoints (U+E1A0…) are shown as U+FFFD (�) inside herdr panes and in the sidebar (agent logo and branch token). The same codepoints render correctly when printed in Windows Terminal outside herdr.

Steps to reproduce

1. Outside herdr, in Windows Terminal, run:

   Write-Host "$([char]0xE1A0) test"

   Result: the icon renders.

2. Open a herdr pane and run the same line. Result: � test.

3. In a herdr pane, run:

   Write-Host "$([char]27)[0m$([char]0xE1A0) normal   $([char]27)[1m$([char]0xE1A0) bold$([char]27)[0m"

   Result: both normal and bold show �.

4. Sidebar: an agent logo token containing U+E1A0 shows �.

Expected

The icons render from the installed font, as they do outside herdr.

Notes

• Starship glyphs from the same font family (U+E0A0 branch, U+F07B folder) render correctly in the same panes, so the failure looks specific to the herdr icon codepoints.
• The preview build from 2026-10-12 still shows the replacement character, so this is not fixed there.
• On macOS with Ghostty the icons render. The herdr-radar plugin writes an explicit font-codepoint-map for these ranges. Windows Terminal has no equivalent option, so Windows relies on system font fallback.

Question

Does herdr draw these codepoints through its own cell model, and does it fall back to U+FFFD for codepoints it considers unknown on Windows? If so, is there a supported way to map them to a specific font?