Backport useDeferredValue initial values

[christopher-buss]

Summary

Backport React 19.2's optional initialValue argument for useDeferredValue on top of #25.

The first render can commit the supplied preview immediately and then schedule the final value as a distinct deferred task. The port also includes the behavior needed to recover suspended previews, avoid nested deferred waterfalls, and preserve the existing one-argument React 18 contract.

React Debug Tools includes the #28508 correction: inspection logs and returns the committed deferred value so the inspected component follows the committed tree's control flow and every subsequent hook stays aligned.

Dependency and consolidation

• Depends on feat(react): backport React 18 transitions #25 and must merge after it. This branch is based directly on commit c6ce564.
• Backport React 19 use API #32 owns the client use API and ReactFiberThenable, including the development/production thenable-state split. This PR neither depends on nor duplicates that separate backport; Anime Rush composes both standalone ports during integration.
• feat(react): backport insertion and Effect Event hooks #28 shares the current-Luau stack parser, missing-ancestor guard, scalar/nil Memo normalization, nil-useMemo regression, and non-optimized Debug Tools test fixture. Whichever PR merges second must drop those duplicate hunks.
• feat(react): backport React 19 client Actions #34 shares the component-wakeable boundaryless-suspension substrate. Spawned-deferred recovery remains specific to this PR. feat(react): backport React 19 client Actions #34 additionally handles Action wakeables thrown by a HostRoot, so consolidation must retain that non-byte-identical extension. Both PRs use the same marked React 17 JND timer adaptation in tests.
• This PR stays draft until its dependency and shared-hunk consolidation are resolved.

Upstream source and test ledger

Pinned stable contract: React v19.2.0.

Upstream source or test	React-Luau target	Status and deviation
Public hook and dispatcher type	ReactHooks.lua; ReactCurrentDispatcher.lua	Direct. Luau cannot distinguish an omitted argument from explicit nil.
Hook implementation and seven DEV dispatchers	ReactFiberHooks.new.lua	Adapted to React 17's lane layout. Hidden-tree/Activity prerendering is unavailable and marked at the source site.
Deferred-lane request	ReactFiberWorkLoop.new.lua	React 17 has no identity-valued Suspense handler stack. The port reconstructs the exact boundary selected by its canonical throw path using the visibility cursor and shouldCaptureSuspense predicate, including fallback-subtree and unstable_avoidThisFallback semantics.
Deferred lane families and root bookkeeping	ReactFiberLane.lua	React 17's nine transition bits keep their numeric values. Seven remain update lanes and the two highest form an independently rotating deferred family.
Separate task and entangled render lanes	ReactFiberLane.lua	React 17 collapses those sets. A constrained reverse selection chooses a pinged deferred task only when its stored entangled parent is also selected and pinged; priority is unchanged.
DidDefer recovery and DidDefer flag	ReactFiberBeginWork.new.lua; ReactFiberFlags.lua	Direct behavior with site-level register-limit and mutually-exclusive flag-reuse deviations. Hydration-only branches are unavailable.
Boundaryless component suspension and HostRoot unwind	ReactFiberThrow.new.lua; ReactFiberUnwindWork.new.lua	Adapted prerequisite for concurrent roots. Interrupted mounted Fibers restore from alternates; new suspended mounts retain hook state.
Concurrent finish and commit readiness	ReactFiberWorkLoop.new.lua	React 17 folds the modern helpers together and explicitly threads spawnedLane through concurrent and separate sync paths.
Debug Tools implementation	ReactDebugHooks.lua	Returns the committed memoized value. React 19 debugInfo and hookSource schema fields are not available in this React 17 Debug Tools model and are marked at the test site.
Public deferred-value cases	ReactDeferredValue.spec.lua	All renderer-supported cases remain in upstream order. React 19 sibling-prewarming duplicate logs are omitted because React 17 does not prewarm siblings.
Debug Tools #28508 regression	ReactHooksInspectionIntegration.spec.lua	Preserves initial, pending, and final rendered output; State; DeferredValue; conditional Context; and both trailing Memo assertions. The slotless renderer read and inspection dispatcher read are inlined so the translation adds no custom-hook frame. React 17 StrictMode replay, synchronous act, Promise construction, one-based IDs, and option naming are marked locally.
React 17 handler reconstruction	ReactDeferredValue.spec.lua	Two local regressions prove a deferred hook in a fallback marks the outer catcher and an avoided inner boundary selects the desirable outer catcher.
Reserved-lane contract	ReactFiberLane.roblox.spec.lua	Six local cases cover matching, unpinged, unrelated-parent, transition-priority, and multiple-deferred selection.
Fizz, hydration, Activity/Offscreen hidden prerendering, and browser popstate tests	No target	Out of scope because React-Luau exposes none of those renderer contracts on this branch.

Behavior-bearing history includes #27500, #27509, #27512, #27888, #28508, #32821, and #34376.

Test-driven regressions

• Before deferred-task reverse selection, the boundaryless recovery case retained the suspended preview yield before Final.
• Before canonical Suspense capture selection, the fallback-subtree case stopped after Primary, Loading, and Outer Fallback instead of attempting Primary and Final again.
• Before canonical avoided-boundary selection, the desirable-boundary case stopped after Loading and Outer Fallback instead of attempting Final.
• Before the React 17 StrictMode setter adaptation, the #28508 test dispatched into the abandoned mount queue and never produced the pending tree.

Each failing state was reproduced before its corresponding implementation and the assertions were retained.

Verification

Exact candidate:

• Commit: dcd0350
• Tree: 6eef706509b8354de08fc0c965a17e5e98192f10
• One commit on dependency head c6ce564
• Archived source snapshot SHA-256: D50B88BE6EB1E7D7BB99F68CE146504027CDF1FDED02CF8E13BA194F8EA5619E

Static checks:

• StyLua 0.18.1 check: passed on all 13 changed Luau files
• luau-compile: passed on all 13 changed Luau files
• Selene 0.28.0: 0 errors, 0 warnings, 0 parse errors
• git diff --check and privacy/debug-residue audit: passed
• Independent provenance, deviation, ancestry, attribution, and source-link audit: passed

Hardened single-runtime Studio CLI:

• Package identity: 1/1
• ReactDeferredValue: 15/15
• ReactFiberLane: 53/53
• ReactHooksInspectionIntegration: 17 passed, 3 pre-existing skipped
• Focused #28508 regression: 1/1
• Focused nil-useMemo regression: 1/1
• Focused fallback-subtree handler regression: 1/1
• Focused desirable-boundary regression: 1/1

Generated with Codex.