enhance(react): useFetch() returns UsablePromise thenable for React.use()

[ntucker]

Motivation

Building on #3752, useFetch() now returns a proper thenable that works directly with React.use(). This makes use(useFetch(endpoint, args)) a drop-in equivalent to useSuspense(endpoint, args) — same suspend, resolve, error, and invalidation behavior — while also enabling parallel data fetching that avoids sequential waterfalls.

Solution

• trackPromise.ts: Replace FetchPromise/RESOLVED/trackPromise() with UsablePromise/createFulfilled()/createRejected() — thenables with status/value/reason fields that React.use() reads synchronously.
• useFetch.ts / useFetch.native.ts: Return fulfilled thenables with denormalized data when cached, rejected thenables on errors, and pending promises when fetching. Add GC tracking via countRef.
• Tests (useFetch-use.web.tsx): Comprehensive test suite cloned from useSuspense.web.tsx and integration-endpoint.web.tsx, validating behavioral equivalence. Gated behind describeIf19 to skip on React 18.
• Docs (useFetch.md): Updated with use() usage, parallel fetch pattern, behavior table for use(), and AsyncBoundary example.
• Existing tests: Updated to validate thenable properties (status, value, then) and denormalized data on fulfilled thenables.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 74d348d

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

This PR includes changesets to release 5 packages

Name	Type
@data-client/react	Major
@data-client/img	Major
@data-client/test	Major
test-bundlesize	Patch
coinbase-lite	Patch

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[github-actions]

Size Change: 0 B

Total Size: 80.6 kB

ℹ️ View Unchanged

Filename	Size
examples/test-bundlesize/dist/App.js	3.42 kB
examples/test-bundlesize/dist/polyfill.js	307 B
examples/test-bundlesize/dist/rdcClient.js	10.2 kB
examples/test-bundlesize/dist/rdcEndpoint.js	6.24 kB
examples/test-bundlesize/dist/react.js	59.7 kB
examples/test-bundlesize/dist/webpack-runtime.js	726 B

_{compressed-size-action}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.05%. Comparing base (3c3bfe8) to head (74d348d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3755   +/-   ##
=======================================
  Coverage   98.04%   98.05%           
=======================================
  Files         151      151           
  Lines        2818     2821    +3     
  Branches      552      553    +1     
=======================================
+ Hits         2763     2766    +3     
  Misses         11       11           
  Partials       44       44           

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