[abhinav]

Removing or reordering an open stack member can require unstacking
and recreating native membership.
Merged downstack PRs prevented that replacement because the compiler
rejected any merged member and execution required an empty resource
after unstacking.
The active suffix stayed in its old stack with stale base branches.

Allow replacement after a merged prefix by recording the historical
merged PRs in the unstack instruction.
Those PRs can remain in the old resource without preventing the released
open PRs from being retargeted and stacked anew.
Queued or auto-merge members still prevent replacement,
and any other retained member stops dependent writes.

[abhinav]

This change is part of the following stack:

• 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

_{Change managed by git-spice.}