fix: inherit deleted/keep flags when splitting an item

[iasakura]

Item::split_at now sets the deleted and keep flags on both halves from the original item, as yjs does in Item.split. Node::split_at had the same issue.

Before this change, splitting a tombstoned item brought the deleted content back, and with gc on every later apply_update failed with JwstCodecError::Unexpected. See test_split_of_tombstoned_item_keeps_both_halves_deleted.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 83.58209% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.93%. Comparing base (1c81ec8) to head (c4a63fc).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
y-octo/src/doc/codec/refs.rs	0.00%	11 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #60       +/-   ##
===========================================
+ Coverage   15.29%   78.93%   +63.64%     
===========================================
  Files           1       59       +58     
  Lines         582    10825    +10243     
===========================================
+ Hits           89     8545     +8456     
- Misses        493     2280     +1787     

Flag	Coverage Δ
node-binding-test	15.29% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.