Use runtime NEON detection for Android armeabi-v7a Opus builds

[coderabbitai]

Summary

Jamulus.pro defines OPUS_ARM_PRESUME_NEON_INTR=1 when QT_ARCH=armeabi-v7a. On 32-bit ARMv7, NEON is not part of the base ISA. This setting can make Opus execute NEON instructions without runtime feature detection.

Required change

For QT_ARCH=armeabi-v7a, do not define OPUS_ARM_PRESUME_NEON_INTR=1 unless the project explicitly excludes non-NEON devices. Retain OPUS_ARM_MAY_HAVE_NEON_INTR=1 and OPUS_HAVE_RTCD=1 so Opus can use runtime dispatch. Continue to define the AArch64-specific preset setting only for arm64-v8a and arm64.

If the project intentionally requires NEON for all ARMv7 Android targets, document that compatibility policy next to the define.

Rationale

On an ARMv7 device without NEON, an unconditional NEON intrinsic path can cause an illegal-instruction crash. Runtime dispatch supports NEON where available without requiring NEON for all ARMv7 devices.

Affected area

• Jamulus.pro
• Android armeabi-v7a Opus build defines
• Opus ARM RTCD configuration

Acceptance criteria

• The armeabi-v7a configuration does not presume NEON unless a documented minimum-device policy requires it.
• Opus retains runtime detection for optional ARMv7 NEON.
• AArch64 configurations retain their appropriate NEON preset defines.

Backlinks

• Pull request: Build opus with NEON intrinsics on ARM (Android ABIs included) #3833
• Review comment: Build opus with NEON intrinsics on ARM (Android ABIs included) #3833 (comment)
• Related closed issue: ARM: Possibly missing opus optimizations #2806
• Requested by: @ann0see

[mcfnord]

🤖 AI: Moot for this APK; only the comment is worth adding.

The #3833 CI build already executes NEON everywhere: every armeabi-v7a compile line is -target armv7a-linux-androideabi21 with no -mfpu, and NDK r21d clang predefines __ARM_NEON 1 for that target. In libJamulus_armeabi-v7a.so, 373 NEON-only instructions sit in Jamulus C++ (Ui_CClientSettingsDlgBase::setupUi alone has 99), none behind a feature check. minSdkVersion 21 plus Google's NEON guide rule out every non-NEON device anyway.

Proposed comment beside the define: # NEON is presumed on armeabi-v7a: minSdkVersion 21 and the NDK r21+ default already make it the baseline for all of libJamulus.

CodeRabbit's evidence trail here is an rg over Jamulus.pro and a gh issue list; the manifest, the NDK pin and the built APK were in the same checkout, unopened. Its rationale is a crash on devices minSdkVersion 21 already refuses, distilled from a review that had conceded "modern Android requires NEON on armv7 (so this is low-risk)" in the same paragraph. It wants opus to ask permission for NEON in a binary whose settings dialog already took 99 without asking.