fix: wrap EditDelta.new_lines in Arc to avoid expensive deep clones

[warp-dev-github-integration]

Description

Wrap EditDelta.new_lines in Arc<Vec<StyledBufferBlock>> to avoid expensive deep clones when the delta is shared across multiple subscribers (e.g. CodeEditorModel → RenderState).

Root Cause

When content changes are emitted via BufferEvent::ContentChanged, the EditDelta (which includes all styled buffer blocks for the changed range) is cloned by each subscriber. For large files or full-buffer invalidations like invalidate_layout, this deep-copies all String data in every StyledBufferRun, leading to massive memory usage.

Heap profile from Sentry issue 7259255054 (9.46GB total in-use):

• 5.80GB (61%): Vec<StyledBufferBlock>::clone in CodeEditorModel::handle_content_model_event
• 0.91GB (10%): DirectoryFetcher::fetch_files_async (separate issue)
• 0.86GB (9%): rebuild_layout_and_refresh_diff layout invalidation
• 0.76GB (8%): Buffer text editing operations

Fix

• Changed EditDelta.new_lines type from Vec<StyledBufferBlock> to Arc<Vec<StyledBufferBlock>>
• Added EditDelta::new() constructor that wraps in Arc
• Updated all construction sites in buffer.rs and core.rs
• layout_delta uses Arc::try_unwrap to avoid cloning when this is the last reference
• Test assertions dereference the Arc for comparison

Linked Issue

• Sentry: https://sentry.io/organizations/warpdotdev/issues/7259255054/events/0e7bc8aa280d475081a61873d5a529a1/

Testing

• cargo check --workspace passes
• cargo clippy -p warp_editor --all-targets passes with no warnings
• cargo check -p warp_editor --tests passes

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

Conversation: https://staging.warp.dev/conversation/dbd6a0a5-bebd-443f-982c-5cb279fed4e7
Run: https://oz.staging.warp.dev/runs/019e5894-9699-7f31-8efb-1eb904aa942b
This PR was generated with Oz.