perf(android): use indexed lookups for legacy batches

[OskarEichler]

Summary

Replace repeated linear searches and list copies in legacy reads/merges with hash lookups.

Compatibility / observable behavior

No intended API or ordering change; legacy deduplication is deliberately preserved.

Related to #1331. This is a focused part of a source review; it does not claim the entire repository is bug-free.

Test Plan

• Compiled actual Kotlin read bodies: 500 randomized equivalence cases preserve database order, appended missing keys, nulls and duplicate-key behavior. An illustrative 10,000-missing-key run was 167 ms before and under 1 ms after (single local run, not a production benchmark). Merge lookup uses the database's unique primary-key invariant; JSON merge behavior is unchanged.
• Upstream yarn build:js, yarn test:ts (6 tasks), yarn test:lint (6 tasks), yarn test:format and git diff --check pass on the combined review checkout.
• No checked-in test/spec files changed. Reproductions were isolated and used synthetic data, not production/user storage.
• The combined 3.1.1 backport passes consumer immutable install/lint, both products' Android Debug and iOS Simulator Debug builds, four production Metro bundles, 13 web targets and four browser-extension targets. Published native framework binaries are unchanged. React Doctor: 100/100.
• Shared-storage testAndroidHostTest, macosArm64Test and iosSimulatorArm64Test pass with Java 21. The first Android run used Java 17 and failed Robolectric's SDK 36/JDK check; rerunning with the project's documented JDK fixed the environment failure without a source workaround.
• Windows/WinRT runtime, signed releases and physical-device behavior have not been verified locally.

[changeset-bot]

🦋 Changeset detected

Latest commit: 57c7755

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

This PR includes changesets to release 1 package

Name	Type
@react-native-async-storage/async-storage	Patch

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