feat: Headless software surface for golden image testing

[kolkov]

Part of gogpu/gogpu#373 — Headless Golden Image Testing (ADR-053), Phase 1.

Summary

Add headless mode to the software HAL backend — render to pixel buffer without requiring a platform window (HWND, X11 Window, Wayland wl_surface).

This is the foundation for golden image testing in CI (ADR-053 in gogpu). GitHub Actions runners have no GPU or display — headless software rendering is the only path for automated visual verification.

Problem

Software backend currently requires a platform surface to render. CreateSurface needs a window handle. CI runners cannot create windows → no visual tests.

Proposed Solution

Add CreateHeadlessSurface(width, height) that:

• Allocates internal BGRA buffer (same as current software surface)
• Supports normal Acquire → Render → Present cycle
• Present writes to buffer instead of BitBlt/SHM/XPutImage
• ReadPixels() []byte extracts rendered RGBA data

The software HAL already has the full rendering pipeline (SPIR-V interpreter, rasterizer, blending). Headless mode only changes the output: buffer instead of window.

Key Files

• hal/software/surface.go — add headless surface variant
• hal/software/present.go — add headless present path (buffer capture)

Testing

• Unit: create headless surface, render solid color, verify pixel values
• Integration: render triangle via standard pipeline, verify output matches expected

Blocked By

Nothing — this is Phase 1 (foundation).

Blocks

• gogpu golden test harness (Phase 2)
• gg 2D rendering goldens (Phase 3)
• ui widget snapshot goldens (Phase 4)