perf: wrap EditDelta.new_lines in Arc to avoid deep-cloning styled blocks

[warp-dev-github-integration]

Summary

Fixes excessive memory usage detected by Sentry issue #7259255054 (82 events, mac_stable_release / mac_preview_release).

Root Cause

The EditDelta struct's new_lines field (Vec<StyledBufferBlock>) was being deep-cloned every time a content-change event was forwarded to the render pipeline. For large files loaded via Buffer::replace_all, this vector contains all styled blocks for the entire file content.

Heap profile analysis showed ~7 GB total in-use memory with the following top allocators:

• 41.1% (3 GB) — Buffer::replace_all → SumTree append_str (buffer text replacement)
• 24.7% (1.8 GB) — Vec<StyledBufferBlock>::clone() in CodeEditorModel::handle_content_model_event
• 16.4% (1.2 GB) — Tree-sitter parsing

Changes

1. Wrap EditDelta.new_lines in Arc<Vec<StyledBufferBlock>> — Cloning an EditDelta now bumps a reference count (O(1)) instead of deep-copying every styled block
2. Use Arc::try_unwrap in layout_delta() — When the layout pipeline consumes the delta (typically the sole owner), it unwraps without cloning; falls back to clone only if there are multiple references
3. Remove unnecessary delta.clone() in DelayRendering::flush_render — The delta was already owned from a consuming iterator, making the clone redundant
4. Update all construction sites to wrap in Arc::new()
5. Update test assertions to dereference Arc for comparisons

Expected Impact

• Eliminates ~1.8 GB of unnecessary allocations per large file load event (the Vec<StyledBufferBlock>::clone path)
• Reduces memory pressure from the flush_render code path (additional unnecessary clone removed)

Testing

• Updated 99 test assertions in buffer_tests.rs to work with the Arc wrapper
• All existing test logic preserved (only dereference operator added)

---

CHANGELOG-IMPROVEMENT:
CHANGELOG-BUG-FIX:
CHANGELOG-NEW-FEATURE:
CHANGELOG-IMAGE:

Conversation: https://staging.warp.dev/conversation/5505471b-acef-408a-92f0-799b1dc2186c
Run: https://oz.staging.warp.dev/runs/019e5971-7b78-7e2a-9e82-b127458e0218
This PR was generated with Oz.