fix: render halfwidth katakana voiced marks

[kazunari-kamata]

Summary

Fix rendering for halfwidth katakana graphemes that use halfwidth voiced or semi-voiced marks, such as ｶﾞｷﾞｸﾞｹﾞｺﾞ and ﾊﾟﾋﾟﾌﾟﾍﾟﾎﾟ.

Root Cause

Ghostty reports halfwidth katakana plus U+FF9E/U+FF9F as a wide cell with a spacer tail. Herdr previously normalized the grapheme through unicode-width, which reports these strings as width 1, so the pane renderer could replace them with blanks.

After preserving the grapheme, the ANSI blitter still treated it as width 1 and could draw the tail cell afterward, overwriting the voiced kana on the host terminal.

Changes

• Preserve halfwidth katakana + halfwidth voiced/semi-voiced mark graphemes when Ghostty reports them as wide cells.
• Keep Ghostty spacer-tail cells as empty symbols instead of converting them to spaces.
• Treat these graphemes as width 2 in the terminal ANSI blitter so full and diff redraws skip covered tail cells correctly.
• Add regression tests for pane rendering, retained dirty patches, and ANSI full/diff redraw behavior.

Validation

• cargo fmt --check
• cargo test halfwidth_katakana_voiced -- --nocapture
• cargo test halfwidth_voiced_kana -- --nocapture

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[kazunari-kamata]

Replaced by #2257 to use a branch name without the agent/ prefix and a scoped commit/PR title matching the project convention.