Harden native material previews and export

[SS-360]

What changed

• Gate node previews, 3D previews, and Blender export on an available Godot rendering device.
• Return CAPABILITY_NOT_AVAILABLE in headless sessions instead of attempting invalid shader rendering.
• Detect stale installed MaterialPilot bridge files before native application tests.
• Replace the UI-oriented Blender export path with bounded rendering of connected Material channels, temporary-file publication, and SHA-256 verification.
• Honor MATERIALPILOT_RUNTIME_FILE in the industrial-floor workflow for isolated bridge sessions.
• Correct the Normal Map recipe and reject effectively flat normal outputs through decoded PNG variance checks.
• Document renderer requirements, stale-addon recovery, export behavior, and the expanded native test matrix.

Why

The production material workflow could show Godot's purple missing-material fallback when rendering was attempted without a real rendering device. Full-resolution Blender export could also wait indefinitely on Material Maker's unrelated global preview queue. A stale installed addon made these failures difficult to distinguish from current source behavior, while structural validation did not detect a flat normal map.

Impact

Live windowed sessions now produce verified sphere/plane previews and Blender-ready 2048 PBR channels. Headless sessions fail rendering operations explicitly and safely. Native integration tests detect source/install drift, and the flagship material workflow validates visual output in addition to graph structure.

Validation

• pnpm format
• pnpm check — 40/40 tasks
• pnpm build — 11/11 packages
• pnpm test:godot
• pnpm test:material-maker in headless mode
• MATERIALPILOT_WINDOWED_TEST=1 pnpm test:material-maker
• Full Abandoned Industrial Floor workflow — 46 stages, 54 artifacts, four strict validation reports, zero workflow warnings
• Independent artifact size and SHA-256 verification
• Normal Material Maker shutdown and bridge-record cleanup