dasVulkan is off by default on Apple builds

[borisbat]

Behavior change: an Apple build leaves dasVulkan out unless configured with -DDAS_VULKAN_DISABLED=OFF; a Mac build directory that had the module keeps it until the cached option is unset (cmake -B build -UDAS_VULKAN_DISABLED).

Why. On a Mac, Vulkan is MoltenVK, whose 32 KB workgroup memory cap declines the kernel classes the module's consumers serve, and what it proves does not carry to a native Vulkan device. Every Mac build paid the module's build time and every dasLLAMA test child its engine-module compile for two permanently red suite files.

What changes.

• The root CMake option DAS_VULKAN_DISABLED defaults to ON on Apple and stays OFF elsewhere; the flag remains explicit for anyone who wants MoltenVK.
• The nightly Vulkan lane's macOS job passes -DDAS_VULKAN_DISABLED=OFF, since proving the MoltenVK loader path is its purpose.
• Preflight's compile sweep skips the four Vulkan-gated trees (examples/vulkan/, examples/vulkan_imgui_cube/, examples/gltf/mesh_viewer_vulkan/, modules/dasVulkan/) while the module's shared artifact is absent, and sweeps them as before while it exists (SWEEP_NEEDS_MODULE in utils/internal/preflight/config.das).
• The dasVulkan README, ARCHITECTURE sec.9 and ROADMAP say macOS is an opt-in.

Observable behavior.

• A fresh Mac configure prints no dasVulkan module and builds no Vulkan target -> the vulkan test cells skip loudly (require ?vulkan), and run.das -- --suite model-free on the M5 Max goes from two red files to none.
• Linux and Windows builds, every per-PR CI lane, the doc lane (Ubuntu, flag explicit) -> unchanged.
• The nightly macOS Vulkan job -> still builds and probes MoltenVK, now by explicit opt-in.

Where to look. The option block in the root CMakeLists.txt, and the build_macos job of .github/workflows/nightly_vulkan.yml.

Validation, claims, ledger

Validation

• On the M5 Max: a reconfigure with the option unset reads DAS_VULKAN_DISABLED:BOOL=ON, the tree builds, and the two formerly red files read test_vulkan_tier 36 passed and test_vulkan_dec_tail 2 passed, 3 skipped (34/2 and 2/3 before, the MoltenVK cap declining the class rails). The full preflight ran once: the fast tier was red on the compile sweep (three Vulkan example roots with no module to require), which the sweep rule fixes; every lane the red tier skipped then ran alone and green - docs (vulkan2rst skips without the module), tests-cpp, tests-interp, tests-jit, tests-aot, utils-tests - and the sweep gate reads 726 roots compiling with the four Vulkan trees held back.
• No per-PR CI lane runs on macOS, so CI cannot observe the flip; the nightly Vulkan macOS job is the lane that does, and it opts in explicitly.

Not done

• A Mac box that built the module before keeps its cached OFF and its built dasModuleVulkan.shared_module until the option is unset and the artifact removed; CMake does not delete either.

[Copilot]

Copilot review overview

🟡 Changes recommended

The new sweep gating currently only considers .shared_module presence in DLL builds, which can still cause compile-sweep failures in static builds where dasVulkan is disabled.

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Review effort: Lite
Findings: 1 · 2

Open (3)

• Static Vulkan-disabled builds still sweep missing module trees · New
• Replace multi-line module comment with a contract or line comment · New
• Fail fast when SWEEP_NEEDS_MODULE key is missing · New

What changed in this PR

This PR makes dasVulkan opt-in on Apple builds by defaulting DAS_VULKAN_DISABLED to ON under APPLE, while keeping non-Apple behavior unchanged. It also adjusts CI and preflight behavior/docs so Vulkan-dependent trees are only built/swept when the module is actually present.

Changes:

• Default DAS_VULKAN_DISABLED to ON on Apple in the root CMakeLists.txt, preserving OFF elsewhere.
• Update the nightly macOS Vulkan workflow job to explicitly opt in with -DDAS_VULKAN_DISABLED=OFF.
• Teach preflight’s compile sweep to skip Vulkan-gated trees when the dasVulkan shared artifact is absent, and document the opt-in in dasVulkan docs.

File	Description
CMakeLists.txt	Makes dasVulkan disabled-by-default on Apple via an APPLE-conditional default.
.github/​workflows/​nightly_vulkan.yml	Explicitly opts the macOS nightly Vulkan lane into building dasVulkan.
utils/​internal/​preflight/​config.das	Adds SWEEP_NEEDS_MODULE and gates sweep roots on module artifact presence.
utils/​internal/​preflight/​tests/​test_changed_set.das	Extends sweep-exclusion tests to cover module-gated trees.
utils/​internal/​preflight/​README.md	Documents the new split between SWEEP_EXCLUDED and SWEEP_NEEDS_MODULE.
modules/​dasVulkan/​README.md	Clarifies macOS usage as opt-in via -DDAS_VULKAN_DISABLED=OFF.
modules/​dasVulkan/​ARCHITECTURE.md	Updates architecture docs to reflect Apple opt-in and rationale.
modules/​dasVulkan/​ROADMAP.md	Updates macOS section to reflect opt-in behavior.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The change cleanly implements the Apple-default disable with explicit opt-in paths and updates preflight/tests/docs to keep local and CI behavior consistent.

Review effort: Lite
Findings: None

Resolved since last review (3)

• Static Vulkan-disabled builds still sweep missing module trees
• Fail fast when SWEEP_NEEDS_MODULE key is missing
• Replace multi-line module comment with a contract or line comment