feat: add png-exr-alpha float PNG false-unpremul witness

[TMHSDigital]

Summary

• Ships examples/png-exr-alpha/ witnessing that float_buffer=True Image.save() to PNG writes RGBA16 and false-unpremultiplies as if associated-alpha, clamping straight-authored dark values at low alpha to white (closed-form RGB err 0.98 at (0.02, a=1/255)).
• Controls: OpenEXR float round-trip (~3e-8), byte→PNG straight RGBA8, and EXR color_mode='RGB' dropping alpha to opaque.
• Wired gallery.json, plugin manifest, smoke workflow, README/AGENTS/CLAUDE counts (21→22), ROADMAP (shipped + light-linking/VSE candidates), generated gallery pages.

API contract

Float images author straight RGBA via pixels. PNG save treats the buffer as associated, does c/a + clamp, stores 16-bit. Mid-tones/darks with c > a reload as white.

Falsification

Temporarily swapped the float→PNG model check to the byte straight-alpha expecter → exit 6, residual 0.9803922. Restored; check passes again.

Visual

Dual easel (PNG-mangled vs EXR-clean). Contact-sheet compared against armature-bend, damped-track-aim, bmesh-gear. EEVEE render locally on Blender 5.1.1.

Test plan

• Check-only on Blender 5.1.1 (live) — measured values in README/CI logs
• Check-only on Blender 4.5 LTS via blender-smoke CI (4.5 not installed locally; 4.4.3 informal pass only — not a 4.5 substitute)
• EEVEE --output render + hero/preview webp
• python scripts/build_gallery.py; inspected generated docs/gallery/index.html + detail <img alt> / witnesses (no dup/truncation artifacts on q16(...))
• validate + manifest/count + drift + security + 4.5/5.1 smoke on PR

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