renderer: add terminal-independent scene worker ABI

[lawrencecchen]

Summary

• split renderer capture from projection with an owned, pointer-free semantic scene
• add bounded full/delta scene encoding and decoding for external render workers
• add a standalone Metal scene renderer that targets IOSurface without creating a PTY, termio, Surface, or canonical terminal
• serialize resolved Ghostty configuration for worker-local renderer setup
• expose canonical selection and search helpers required by remote terminal interactions
• label worker Metal command buffers, encoders, and textures for process-scoped trace proof

Dependency

cmux integration: manaflow-ai/cmux#8378

Verification

• 4,749 lib-vt tests exercised; only the pre-existing PageList viewport-offset failures reproduced in both static and shared modes
• focused scene codec, materialization, config serialization, sparse color, selection/search, and allocation-failure tests pass
• real standalone Metal IOSurface smoke passes
• universal signed cmux renderer helper exports the scene renderer ABI and completes its authenticated child handshake
• C header syntax, Zig formatting, and diff checks pass

Draft while cmux completes leases, durable topology state, advanced Kitty/custom-shader parity, tagged restart tests, and process-scoped Metal profiling.

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Summary by cubic

Adds a terminal‑independent semantic scene renderer ABI with a standalone Metal renderer targeting IOSurface, plus macOS‑only GhosttySceneRendererKit for out‑of‑process workers and an isolated GhosttyConfigKit for daemon config. Also instruments process‑lifetime counters for runtime app construction, PTY‑master opens/allocations, and surface constructors for external observability.

• New Features

  • Completed GhosttyRenderSceneEncoder with bounded full/delta codec and C APIs in ghostty/vt/render_scene.h.
  • Exposed standalone worker C ABI in ghostty_scene.h (ghostty_scene_init, frame leases/metadata, health events); shipped GhosttySceneRendererKit.xcframework.
  • Added config‑only C ABI in ghostty_config.h (ghostty_config_init, recursive load/finalize, ghostty_config_serialize); shipped GhosttyConfigKit.xcframework.
  • Added Metal scene init without CALayer with labeled GPU trace; Kitty capture/replay (incl. animation), sparse OSC 8 hyperlink capture, selection/search helpers, and ghostty_key_from_macos_keycode.
  • Added smoke/audit tests for the scene renderer and config kits.

• Migration

  • New build flags: emit-lib-vt-shared (native only), emit-scene-xcframework, and emit-config-xcframework.
  • Static xcframework library renamed to libghostty-internal.a.
  • Embedders should:
    • Use ghostty_scene_renderer_* from ghostty_scene.h for worker rendering (and call ghostty_scene_init).
    • Use ghostty_render_scene_* from ghostty/vt/render_scene.h to capture/encode terminal scenes for workers.
    • Use ghostty_config_* from ghostty_config.h for daemon config resolution and serialization.
    • Replace custom macOS keycode tables with ghostty_key_from_macos_keycode.

^{Written for commit 0e750d5. Summary will update on new commits.}

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