[FEAT-2] Native diff applier: Android (jbsdiff) — implement, wire, device-test

[prasad-rently]

Part of FEAT-2 (PR #1). Critical path to diffs working on a phone.

android/.../CodePushDiffPatcher.java is a compile-clean placeholder (applyPatch throws → callers fall back to full download).

To do (see FEAT-2-NATIVE-INTEGRATION.md):

• Add implementation("io.sigpipe:jbsdiff:1.0") to android/build.gradle.
• Implement applyPatch(source, patch) via io.sigpipe.jbsdiff.Patch.patch(...); set IS_INTEGRATED = true.
• Wire into CodePushUpdateManager download path: when diffUrl+sourceHash present and on-disk hash == sourceHash, download patch → apply → verify reconstructed hash == packageHash (CodePushUpdateUtils) → else full-download fallback.
• JUnit/instrumented test: server patch applies → bytes == target; corrupt → fallback.

Acceptance: device on v_n applies a KB-scale patch, boots v_n+1; any failure falls back to full; classic path unchanged.