Build verified AArch64 Grease runtime for tablet Cat Food

[isomorphisms]

Add a cloud-built AArch64 Android/Termux Grease runtime alongside the existing ARMv7 phone artifact.

• reuse the exact Grease/Oils source pin from the phone runtime branch;
• cross-build with the Android NDK for arm64-v8a / AArch64 at API 28;
• omit readline and statically link the C++ runtime as on the phone lane;
• require the resulting ELF machine to be AArch64 and reject libc++_shared.so;
• package bin/grease, its bin/ysh engine, and provenance receipts;
• upload the exact artifact and publish a digest-addressed prerelease on branch push;
• keep physical-tablet execution explicitly pending.

This is intentionally stacked on the ARMv7 runtime PR so the tablet delta is only the AArch64 build/packaging lane. A tablet PASS must not be used as an ARMv7 phone receipt.