build(#1580): ship the arm64-v8a/x86_64 Android JNI model host in the AAR (Decision 108, slice 3/3)

[enricopiovesan]

Summary

Slice 3 of 3 for #1580 (Decision 108): the published traverse-embedder AAR now ships the native model engine (libtraverse_android_host.so) for arm64-v8a (phones) and x86_64 (emulators). Without it, every model call on a device fails closed with engine_unavailable.

• scripts/build_android_host_ndk.sh cross-builds traverse-android-host with cargo-ndk 4.1.2 at API 28 (the embedder's minSdk) into <jniLibs>/<abi>/. It uses the NDK from ANDROID_NDK_HOME / ANDROID_NDK_LATEST_HOME, which GitHub runners provide. It links with 16 KB page alignment (-z max-page-size=16384, required for Android 15+) and then verifies each library:
  • it exports exactly Java_dev_traverse_embedder_ExactModelNative_modelCall, the one audited JNI method from ADR-0079;
  • every LOAD segment is aligned to 0x4000.
• Gradle: sourceSets["main"].jniLibs.srcDir(build/jniLibs), so the libraries ship inside the AAR and nothing binary is committed.
• scripts/ci/android_aar_check.sh fails if the AAR lacks either ABI.
• CI:
  • a new android-native-package job (Ubuntu, Rust Android targets, cargo-ndk) cross-builds, assembles the release AAR, and runs the AAR check on every full-CI PR;
  • the Maven publish job cross-builds and verifies the AAR before publishToMavenCentral.
• Docs (embedder table) and release notes are updated, and Spec 138 governs gains both scripts.

This machine has no Android NDK, so the cross-build itself is proven by this PR's new android-native-package job. That was the CI-only option from the #1580 discussion.

Governing Spec

• 138-governed-exact-model-execution
• 068-public-platform-embedder-packages
• 004-spec-alignment-gate
• 065-sigstore-bundle-verification

068 covers the public Kotlin package build (build.gradle.kts) and the Maven publish workflow. 004 covers the approved-specs.json governs additions and the CI scripts. 065 covers spec-alignment-gate coverage of docs/releases/next.md.

Project Item

Closes #1580 (slice 3/3; slices 1–2 were #1613 and #1614). Emulator instrumented tests remain #1611.

Validation

• Gradle wiring, checked locally with placeholder libraries in build/jniLibs: gradle :traverse-embedder:assembleRelease packs jni/arm64-v8a/… and jni/x86_64/…, and android_aar_check.sh passes. With one ABI removed, it fails with missing jni/x86_64/libtraverse_android_host.so.
• bash -n on both scripts; both workflows parse as YAML; the release naming check passes.
• The real cross-build, export and alignment checks run in this PR's android-native-package job.

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