Investigate ambient HRX in-range decode fault

[Copilot]

The reported decode fault appears ambient rather than specific to the multipass path, but the supplied measurements do not establish a verified residency fix.

• Scope: The transient allocation is in third_party/llama.cpp; the allocation API and HIP/VMM behavior are in third_party/hrx-system.
• Outcome: No code or submodule pins were changed. Both submodule worktrees are empty, and the required runtime behavior has no verified implementation to opt into.
• Open question: Sequential faults did not reproduce at the reported rate on the current pin; the concurrent case remains unverified.

• Fixes Ambient in-range decode fault (2049-32768): the fallback faults >= the multipass; needs libhrx transient pinning #301

[bong-water-water-bong]

done