Add --opaque flag to screenshot for shaders that write alpha=0

[arghhhhh]

Problem

Many TD shaders write fragColor with alpha = 0 — atmospheric/sky shaders that use alpha as a compositing-flag, GLSL templates that simply omit the alpha component, etc. The captured PNG is technically correct (RGBA where A=0) but most image viewers composite RGBA over white when alpha=0, producing a blank-looking image and hiding the actual RGB content.

This is especially confusing because TD's own node-viewer thumbnail ignores alpha and shows RGB directly — so the artist sees a perfectly-rendered sky in the network while the saved PNG looks completely white.

Concretely: I'm using td-cli to drive an LLM agent's visual feedback loop. The agent was repeatedly drawing wrong conclusions about a sky shader because every td-cli screenshot came back blank-white in its viewer, even though the shader was working fine. Sampling pixel values via td-cli exec confirmed real RGB data with alpha=0.

Fix

New --opaque flag on screenshot:

td-cli screenshot /project1/sky/glsl1 --opaque -o sky.png

Implementation:

• Go-side post-processing — decode the PNG returned by the handler, force alpha to 255 on every pixel, re-encode.
• RGB channels preserved as-is (no premultiplied-alpha unpacking — matches what TD's node viewer displays).
• Zero TD-side / handler changes, so it works against every existing handler version.
• Default behavior unchanged.

Test plan

• Builds clean with go build ./cmd/td-cli/
• Verified against a live Nishita sky shader (fragColor = vec4(scattering, 0.0)) — default capture renders white in viewers, --opaque reveals the real RGB
• Default screenshot (no flag) produces byte-identical output to pre-patch
• Handler-side path untouched, but worth a sanity check on a TOP that already writes alpha=1 to confirm --opaque is a no-op for it

Notes

Help text updated in cmd/td-cli/main.go. Did not touch internal/commands/init.go (the generated CLAUDE.md) — happy to add a line there if you'd prefer.

🤖 Generated with Claude Code