Investigate Linux x64 Vulkan LoRA mismatches after Ubuntu 26.04 toolchain drift #37

Linux x64 Vulkan qualification began failing after the floating runner moved from Ubuntu 24.04 to 26.04. PR #36 pins qualification and release builders to Ubuntu 24.04 and passes all 17 checks; this issue retains the unresolved numerical/toolchain investigation.

Evidence

• Passing baseline: run 40550578566, Ubuntu 24.04 image 20261012.334.1, Mesa 25.2.8/LLVM 20.1.2, Vulkan 1.3.275, glslc 2023.8.
• Failing PR Reject unsafe VRAM-budget loads before context creation #35: job 125305159695, Ubuntu 26.04 image 20261025.179.1, Mesa 26.0.8/LLVM 21.1.8, Vulkan 1.4.341, glslc 2026.1. The software GPU newly reports KHR_coopmat.
• Twelve F16/BF16 linear and linear-FP8 LoRA immediate/auto comparisons against at_runtime fail with MAD around 1.37 against the unchanged 0.5 threshold. Build/export/device checks pass. These probes do not set max_vram, so PR Reject unsafe VRAM-budget loads before context creation #35's new rejection branch is bypassed.
• Controlled Linux arm64 baseline/candidate builds on Ubuntu 24.04 produce identical images. The same candidate binary on Ubuntu 26.04 also matches, MAD 0.11637. Those controls lack cooperative matrices and do not clear the failing x64 path.
• PR #36 run passes the pinned Ubuntu 24.04 x64 Vulkan comparisons. No threshold or cooperative-matrix setting was disabled.

Acceptance

Separate source, compiler/SPIR-V and runtime/driver effects with exact same-source x64 controls; isolate the failing operation/path before assigning upstream or driver causality. Keep the numerical threshold and positive controls. Qualify an eventual Ubuntu/toolchain upgrade before changing the pinned producer and qualification environments together.

No PR-caused runtime regression or hardware-GPU failure is established. Failed-run artifacts were not uploaded, so no GLIBC import-floor claim is made. Related owner qualification umbrella: #20; release tracking: leehack/llamadart#1703.