forge: Add native stack updates

[abhinav]

Forge repositories can expose native stacking through the optional
WithStacks upgrade. GitHub projects the complete open change graph
onto a deterministic linear same-repository path, then creates or
extends the native stack on a best-effort basis.

GitHub cannot replace an existing native stack with a divergent path.
Constrain path selection with compatible native membership before
scanning its upstack, so a longer unstacked branch cannot displace a
manually chosen native path. Leave multiple or incompatible native
stacks unchanged and warn once for the affected tree.

Use the compact gateway projection for reconciliation. The shared forge
integration suite covers stack creation, extension, and idempotence.

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.}