merge: Use native ranges when available

[abhinav]

Merge selected linear paths atomically when a repository implements
WithMergeRange. Prepare each change and verify its published base,
head, and commit before scheduling the operation, then use the existing
timeout for asynchronous acceptance and final change-state polling.

Treat runtime ErrUnsupported as a signal to resume the ordinary
bottom-up merge sequence. Divergent heads remain separate queue
dependencies, so one native linear path can merge without consuming an
unselected sibling.

ShamHub constructs merge and squash range results before one
compare-and-swap target update. Propagate request cancellation through
validation, commit construction, and publication so a canceled operation
cannot advance the target or mark changes merged.

A GitHub-only integration scenario registers A-B-C as a native stack,
merges it atomically, and verifies that divergent D remains open,
unstacked, and mergeable on B with its head branch and commit unchanged.
Its cassette remains deliberately unrecorded for a human operator to
capture.

[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.}