[darkwebber]

Opening as a draft to discuss before anyone reviews it in detail: it's bigger than a usual change and touches GPUI. Ghostty users often have custom-shader files (cursor trails, glow, CRT); this lets a layout run the same files over the Herdr window, unchanged.

What it does

• Herdr: a layout root takes "shaders": [paths] (relative to the app's folder, or ~/). src/shaders.rs supplies Ghostty's prelude and uniforms (iChannel0, iTime, iResolution, iCurrentCursor / iPreviousCursor / iTimeCursorChange, iCurrentCursorColor, iFocus / iTimeFocus, iPalette, the theme's colors). Shaders reload when their files change, compile errors show in a toast, and frames keep coming while the window has focus (like custom-shader-animation = true).
• GPUI: a small API that knows nothing about Ghostty. Window::set_post_process_shaders(&[glsl]) takes GLSL 450 fragment shaders, and set_post_process_values fills their uniform block by member name. The Metal backend translates them with naga (glsl-in, msl-out), draws the scene to an offscreen frame and runs the passes into the drawable. Other backends report that post-processing isn't supported yet; wgpu could do the same with naga's WGSL output.
• Uniforms are written at the offsets the generated Metal struct really uses: naga lays the block out as std140 but writes float[4] packed, so offsets after scalar arrays are corrected (with a test).
• THIRD-PARTY-NOTICES is regenerated for naga's GLSL parser, and defaults/AGENTS.md documents shaders.

Before growing the core

1. The app acts on it: it changes how every frame is drawn.
2. Only the app (and GPUI) can see the frame.
3. A mod can't draw over the whole window or read the frame.

Open questions

• Whole-window pass (this) vs per terminal (Ghostty's are per surface).
• Power: frames run continuously while focused. Should there be a setting to animate only after cursor or focus changes?
• macOS only for now.

Testing

• cargo test --locked passes; there are new tests in shaders.rs (the prelude with a typical cursor shader validates in naga, palette) and in post_process.rs (uniform offsets).
• Running on macOS (Apple Silicon) with four real Ghostty shaders, cursor_blaze_no_trail, cursor_smear, a ripple on cursor jumps and an ambient bloom/vignette: trails follow the cursor, and the unfocused window dims.

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