Some recommended test scenarios:

• Update chain that adds data to a placeholder checkpoint in the original chain.
• Update chain which has a PoA with the original chain at a placeholder checkpoint (and vise-versa).

With the above tests:

• Ensure the resultant chain has no "floating" checkpoints.

@LagginTimes please review the modifications I made to push and insert here: evanlinjin@1939dd2

This implementation builds upon and improves the original PR's approach to optional checkpoint data in several key ways:

1. Cleaner insert logic
   The original PR had complex nested conditions for handling insertions. This version:

   • Splits the chain into base and tail components upfront
   • Uses a systematic rebuild approach that leverages push for validation
   • Handles all edge cases (displacement, purging, placeholder filling) in a unified flow

2. Proper displacement handling

   • Clear distinction: When a prev_blockhash conflicts, the checkpoint is displaced (converted to placeholder) rather than purged, preserving the chain structure
   • Orphan management: Checkpoints that become orphaned after displacement are correctly purged
   • The original PR struggled with maintaining consistency when conflicts occurred at different points in the chain

3. Placeholder cleanup in push

   • Added logic to remove trailing placeholder checkpoints before pushing new data
   • Prevents accumulation of unnecessary placeholders at the chain tip
   • Maintains cleaner chain structure over time