feat: add sky-texture-sun-elevation example

[TMHSDigital]

Summary

• New gallery example sky-texture-sun-elevation: World ShaderNodeTexSky → Background, version-gated NISHITA (4.5) vs MULTIPLE_SCATTERING (5.1), dust_density vs aerosol_density, and sun_elevation zenith rise proven with two tiny Cycles OPEN_EXR probes.
• Gallery still is a dual-elevation diptych (8° | 55°) so the contract reads at thumbnail scale. Stage deviation documented (sky is the contract).
• Counts 32→33; wired into gallery.json, plugin manifest, blender-smoke, README, ROADMAP; contact sheet committed.

Contract witnessed (live run)

Binary	Path	Reported	zenith_rise
5.1.2	.scratch/blender-5.1.2-windows-x64/blender.exe	(5, 1, 2) / 5.1.2	2.2546 (gate ≥ 1.25)
4.5.11 LTS	.scratch/blender-4.5.11-windows-x64/blender.exe	(4, 5, 11) / 4.5.11 LTS	1.4958 (gate ≥ 1.25)

Falsification (live run on 5.1.2)

• Unlink Sky→Background → exit 6 (world links broken)
• Raise ZENITH_RISE_MIN to 99.0 → exit 8 (rise=2.2546 < 99.0)
• Restored after each probe

Visual passes

• Iterations: 8 (washed single-frame sky → tighter framing → diptych redesign; labels + contact sheet)
• Pass 1: Standard transform, terracotta materials, diptych framing, no helpers
• Pass 2: panels differ when elevation differs (panel ΔL1 ≈ 0.10); identical panels would mean elevation failed
• Pass 3: preview webp reads as two skies + jar at card size
• Pass 4: Playwright on python -m http.server from docs/ — desktop 1280×4000 (33 cards, sky visible), mobile 390, detail page shows NISHITA + 2.25
• Pass 5: contact sheet — meanY vs cal set: armature-bend 0.277, damped-track 0.174, bmesh-gear 0.305, candidate 0.270. Deviation: Nishita/MS sky diptych (not default studio). Judge materials/framing/Standard/thumbnail — not sortable as a different gallery.

Evidence labeling

• Live run: both Blender checks, falsification exits, still render, Playwright screenshots, contact-sheet luminance
• Inspection only: generated HTML alt/witnesses scan after build_gallery.py

Test plan

• blender-smoke 4.5 LTS + 5.1 both green on PR head
• validate + drift-check green
• Socket Security resolved (wait if pending)
• Local: .scratch/blender-5.1.2-windows-x64/blender.exe --background --python examples/sky-texture-sun-elevation/sky_texture_sun_elevation.py --
• Local: .scratch/blender-4.5.11-windows-x64/blender.exe --background --python examples/sky-texture-sun-elevation/sky_texture_sun_elevation.py --