fix(terminal): 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.

Docs

No public docs update needed. This is a terminal rendering bug fix and does not change documented commands, configuration, or workflows.

Validation

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

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[greptile-apps]

Greptile Summary

The PR preserves Ghostty’s halfwidth voiced-katakana graphemes and spacer tails through pane rendering, then accounts for their effective two-cell width in ANSI output.

• Preserves halfwidth katakana followed by voiced or semi-voiced marks when Ghostty marks the grapheme wide.
• Keeps SpacerTail symbols empty in full and retained pane data.
• Updates ANSI full and differential redraw width handling.
• Adds pane, retained-patch, and ANSI regression coverage.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/pane/terminal.rs	Preserves halfwidth voiced-katakana graphemes reported as wide and retains empty Ghostty SpacerTail symbols through pane and dirty-patch rendering.
src/protocol/render_ansi.rs	Treats the targeted grapheme pair as two cells so full and differential ANSI redraws avoid overwriting its covered tail.
src/server/headless.rs	Adjusts frame-comparison test helpers to recognize empty and space tail symbols as equivalent only when covered by a preceding wide cell.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  PTY[PTY bytes] --> Ghostty[Ghostty terminal]
  Ghostty --> Head["Wide kana head: ｶﾞ"]
  Ghostty --> Tail["SpacerTail: empty symbol"]
  Head --> Pane[Pane / retained frame]
  Tail --> Pane
  Pane --> ANSI[ANSI blitter]
  ANSI --> Width[Effective width = 2]
  Width --> Host[Host terminal redraw skips covered tail]

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_{Reviews (2): Last reviewed commit: "Merge branch 'master' into fix/halfwidth..." | Re-trigger Greptile}

[ogulcancelik]

ty!