[abhinav]

Forge repositories need an optional operation that can atomically merge
a fully restacked linear change range while rejecting stale remote
state. Add WithMergeRange with the expected branch names and head
commits needed for providers to enforce that boundary.

GitHub keeps its full pull request projection local to range validation,
checks branches, commits, and native-stack membership, then delegates to
the asynchronous merge gateway. The shared integration scenario creates
the required native stack before starting the range merge and waits for
operation acceptance and final merged states.

The GitHub integration cassette remains deliberately unrecorded for a
human operator to capture.

Refs #1388

[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
          • submit: Update native stacks after publishing #1400
            • merge: Use native ranges when available #1401
              • submit: Finalize partial submissions #1403
                • docs: Explain native GitHub stacks #1406

_{Change managed by git-spice.}