[abhinav]

Document automatic native-stack registration after submit, structural
reconciliation on later submissions, and atomic range merging.
Explain provider fallback and the spice.forge.github.stacks opt-out.

Describe why composition changes may dissolve a native stack before pull
request bases change, and note that queued or auto-merge-enabled pull requests
can prevent GitHub from restructuring the stack.

Resolves #1388

[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
      • merge: Use native ranges when available #1401
        • docs: Explain native GitHub stacks #1406 ◀

_{Change managed by git-spice.}