Fix several small performance inefficiencies in hot paths

[miguelraz]

Objective

Land five small, independent performance fixes found during an audit of hot paths (anchor resolution, line shaping, worktree scanning, and sorting in multibuffers).

Solution

Each fix is its own commit, so this PR is best reviewed commit-by-commit — every commit message contains the full reasoning for that change:

• text: Avoid redundant rope traversal in Anchor → usize conversion — call offset_for_anchor directly instead of summary_for_anchor, which recomputed the same byte offset with ~4 extra O(log n) tree walks. This is the hottest anchor-resolution path.
• editor: Avoid double allocation per shaped line — line.as_str().into() instead of line.clone().into(), which allocated twice per visible line, every frame.
• text: Use sort_unstable_by_key in operation queue insertion — Lamport timestamps are unique keys and duplicates are deduped right after, so stability buys nothing.
• multi_buffer: Sort with an explicit comparator instead of sort_unstable_by_key, which cloned a PathKey (Arc refcount bump) on every comparison.
• worktree: Replace O(n²) Vec::remove in the deferred-directory pass with an O(1) None assignment — the vec is already Vec<Option<ScanJob>> and is consumed with .flatten().

Testing

• No behavior changes intended; all changes are mechanical and tests affected crates pass: text, editor, multi_buffer, worktree.

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 (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

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Release Notes:

• Improved editor performance through several micro-optimizations in anchor resolution, line shaping, and worktree scanning.