feat(health_monitor): implement ARM MTE memory provider

[SebStefenel]

Summary

Implements the ARM MTE memory provider requested in #119: ProtectedMemoryAllocator allocates memory regions protected by the ARM Memory Tagging Extension. The provider is gated behind the new //config:enable_arm_mte Bazel flag (default: off) and the mte Cargo feature, so it can be enabled once the target hardware and OS support it.

Behavior by configuration:

• Flag off (default): regions are plain, zero-initialized, 16-byte aligned memory served by the Rust allocator (portable across Linux and QNX).
• Flag on, aarch64 Linux: regions are mapped with mmap(PROT_MTE), MTE support is detected at runtime (getauxval(AT_HWCAP2)), synchronous tag checking is enabled per thread (prctl(PR_SET_TAGGED_ADDR_CTRL)), and a random non-zero tag is assigned to every 16-byte granule.
• Flag on, any other target, or a CPU/kernel without MTE: the provider logs an error and degrades to plain memory.

Degrading instead of failing

Allocation never fails because MTE is unavailable. The alternative -- returning an error -- would make the flag unusable on the x86_64 hosts that run the unit tests and the sanitizer builds, and would turn a capability mismatch into a startup failure on targets that are otherwise fine.

Degrading is deliberately not silent, so a caller that must not run unprotected can detect it:

• ProtectedMemoryAllocator::is_protection_active() reports whether the platform provides protection.
• ProtectedMemoryRegion::is_protected() reports whether that specific region is hardware-tagged.
• Every degraded allocation logs an error.

Happy to switch to a hard failure when the flag is explicitly enabled if that matches the safety concept better.

Implementation notes

• The core::arch::aarch64::__arm_mte_* intrinsics are nightly-only (stdarch_aarch64_mte, Tracking Issue for AArch64 MTE memory tagging intrinsics rust-lang/rust#129010), so irg and stg are emitted as inline assembly from functions carrying #[target_feature(enable = "mte")]. This keeps the backend buildable with the Ferrocene toolchain.
• PR_SET_TAGGED_ADDR_CTRL is 55 (56 reads the setting back), PR_TAGGED_ADDR_ENABLE is bit 0, and the control word sets the tag inclusion mask. The kernel reads that field as the set of tags irg may generate, so leaving it zero would exclude all 16 tags and yield tag 0 only.
• //score/health_monitor/src/rust:health_monitoring_lib_mte builds the MTE backend unconditionally for aarch64 Linux, and is skipped elsewhere. Without it no CI configuration compiles that code, since it is gated on both the feature and the target.
• ProtectedMemoryAllocator remains a unit struct, so all existing construction sites (builders, tests) are unchanged.
• Monitor builders keep receiving &ProtectedMemoryAllocator unchanged; moving monitor state into protected regions is intentionally left for a follow-up.
• Known MTE limitations are documented in the module docs (16 tags, per-thread tag-check configuration).

Closes #119