feat(plugin-activity-guard): add activity entry guards

[ENvironmentSet]

Summary

• define the Activity Guard contract and architectural decisions for guarding new Activity entries
• add @stackflow/plugin-activity-guard with typed redirects and ordered and / or combinators
• preserve push, replace, and fresh initial-entry semantics while re-guarding redirect destinations and leaving reactivation, snapshot load, and step navigation unchanged
• add strict type/runtime/React integration coverage, package documentation, and a changeset

Verification

• yarn build
• yarn typecheck
• yarn lint
• yarn workspace @stackflow/plugin-activity-guard typecheck
• yarn workspace @stackflow/plugin-activity-guard build
• yarn workspace @stackflow/plugin-activity-guard test --runInBand
• yarn workspace @stackflow/plugin-activity-guard verify:contract-map
• yarn workspace @stackflow/plugin-activity-guard verify:type-teeth

[changeset-bot]

🦋 Changeset detected

Latest commit: d3f39c0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@stackflow/plugin-activity-guard	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/fep-2521

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[pkg-pr-new]

• @stackflow/demo

  yarn add https://pkg.pr.new/daangn/stackflow/@stackflow/plugin-activity-guard@728.tgz
  

commit: d3f39c0

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	stackflow-docs	d3f39c0	Commit Preview URL	Jul 13 2026, 12:24 AM