[abhinav]

An internally aligned native stack can merge after trunk advances.
Preparing its trunk-based bottom instead rebases that branch and
forces every member above it to restack and resubmit before merging.

Keep the preflight head for a member originally based on trunk,
matching individual merge preparation.
Continue preparing other members, including range bottoms whose bases
were merged by earlier queue items.

Allow the same case in ShamHub by separating the current target head
from the bottom change's merge base.
Apply the stack's delta from that merge base onto the current target
so merge and squash operations preserve independent trunk changes.
Keep the target head as the publication parent and CAS expectation.

[skip changelog]: Fixes unreleased native stack merging.

[abhinav]

This change is part of the following stack:

• submit: Preserve scope in native stack updates #1486
  • merge: Separate execution responsibilities #1484
    • mergequeue: Await preparation results #1488
      • github: Add native stack gateway operations #1396
        • forge: Add native stack updates #1397
          • forge: Add linear merge range capability #1398
            • github: Add native stack opt-out #1399
              • shamhub: Model native stack operations #1405
                • github: Rebuild native stacks after merged prefixes #1478
                  • submit: Update native stacks after publishing #1400
                  • merge: Use native ranges when available #1401
                  • docs: Explain native GitHub stacks #1406
                  • merge: Preserve native stacks behind trunk #1492 ◀

_{Change managed by git-spice.}