terminal: Respect word boundaries after tree branches

[macayu17]

Objective

Fixes #61767.

Double-clicking a service name after a systemctl tree branch also selects the └─ prefix.

Solution

Add ─ to Alacritty's semantic escape characters for both PTY and display-only terminals. The regression test checks that selecting └─zms-demo.target returns only zms-demo.target.

Testing

• cargo fmt --all -- --check
• git diff --check
• Added semantic_selection_stops_at_tree_branch

The regression test could not run locally because this Windows environment does not have MSVC's link.exe. It can be run with:

cargo test -p terminal semantic_selection_stops_at_tree_branch

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

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Release Notes:

• Fixed terminal word selection around tree branch glyphs.

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[Copilot]

Pull request overview

This PR fixes terminal double-click semantic selection so that service names following systemctl-style tree branch prefixes (e.g. └─) are selected without including the branch glyphs, improving copy/paste usability in the terminal.

Changes:

• Extend Alacritty terminal config semantic_escape_chars to treat the box-drawing horizontal bar ─ as a semantic word boundary (for both PTY and display-only terminals).
• Add a regression test asserting semantic selection on └─zms-demo.target returns only zms-demo.target.

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[macayu17]

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