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ARM64 pinned core NEON warning blocks zero-warning soft-float builds #945

Description

@ryanbreen

The pinned nightly-2025-06-24 ARM64 soft-float build completes but reports a future-incompatibility warning from Rust's core, preventing the repository's zero-warning quality gate from being satisfied.

Reproduction:

cargo build --release --features boot_tests --target aarch64-breenix-kernel.json -Z build-std=core,alloc -Z build-std-features=compiler-builtins-mem -p kernel --bin kernel-aarch64
cargo report future-incompatibilities --id 1 --package core@0.0.0

The report identifies #[target_feature(enable = "neon")] in the pinned toolchain's core_arch/src/arm_shared/neon/generated.rs: enabling NEON on this soft-float target is reported as unsound due to ABI issues. This also appears in the inherited PR 3 draft build log and is not introduced by the guest-budget predicate.

Follow-up: validate a compatible toolchain/core update while retaining the soft-float target and the no-NEON binary gate. Do not hide the warning with a blanket lint suppression. PR 3 verification is waiting for an explicit exception or this prerequisite fix.

Refs #586.

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