[leehack]

Summary

Pin both Linux x64 CPU/Vulkan lanes in validation and publication workflows to Ubuntu 24.04. The floating runner moved from Ubuntu 24.04/Mesa 25 to Ubuntu 26.04/Mesa 26 between the last passing baseline and PR #35's LoRA image mismatch. This keeps producer and qualification environments explicit while the mismatch is investigated; it does not disable cooperative matrices or loosen output thresholds.

Separate prerequisite for #35 and 0.18.0 plan. No runtime source, permissions or publication approval gates changed.

Validation

• Three workflow tests pass; the new test fails against prior workflow values in all four target/workflow combinations.
• Independent blocking review clear at 8e44f0f9edf7f5233749a162009a7618d3c5fe13, tree e9f22bc36a4b632cebb3f110a8a95320ac7eb7b9, against 54e97df3b8bfa52f18074bbcc6dab557b323b243.
• All 17 current-head CI checks pass, including Linux x64 Vulkan LoRA/FP8 comparisons, normal/ASAN teardown and the Apple XCFramework consumer. Zero review threads; exact head/base unchanged.

Limits

The runner change coincides with the failure but causality is not established. Same-binary Linux arm64 controls on Mesa 25 and 26 both passed; the failing CI was x64 with a different cooperative-matrix path. This PR does not claim a numerical runtime fix or a measured GLIBC compatibility floor. Actual artifact execution/import qualification remains required. No publication is authorized here.