fix(react): ensure useDocumentProjection reuses state source for all strings

[cngonzalez]

Description

useDocumentProjection was actually a bit finicky about the projection being passed to it. We had a ticket about it looping forever

My best guess about what was happening was this:

1. Any helper functions, or even template literals, created new references passed to useDocumentProjection
2. useDocumentProjection would then call getProjectionState. It always returns a new StateSource, which returns a new subscribe function
3. useSyncExternalStore receives the new subscribe function, triggering a re-render
4. re-render causes the loop to start all over again

This PR adds some memoization to maintain consistency in projection strings and state sources.

We didn't catch this in casual clicking around because defineProjection was doing some of the memoization work for us, I think. To address this, I added some common ways people would use useDocumentProjection and also some e2e tests against it.

What to review

Any risks of over memoization or staleness?

Testing

e2e tests (I think this is hard to capture in React tests and we still have 100% coverage).

To see the OLD behavior,

1. git fetch
2. git checkout main
3. git checkout origin/fix/sdk-660/ensure-doc-projections-reuse-state-source apps/kitchensink-react/src/DocumentCollection/DocumentProjectionRoute.tsx
4. pnpm dev
5. go to https://www.sanity.io/@oblZgbTFj?dev=http%3A%2F%2Flocalhost%3A3333
6. go to https://www.sanity.io/@oblZgbTFj/application/__dev/document-projection
7. Click around some of the many buttons, you'll see things hanging.

This PR fixes that.

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Project	Deployment	Preview	Comments	Updated (UTC)
sdk-docs	Ready	Preview	Comment	Nov 14, 2025 7:20pm
sdk-kitchensink-react	Ready	Preview	Comment	Nov 14, 2025 7:20pm

[cngonzalez]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(react): ensure useDocumentProjection reuses state source for all strings #669 👈 (View in Graphite)
• fix(e2e): ensure tests run in dashboard and correct org when not in CI #666
• main

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[ryanbonial]

Fix works well. Thanks!