feat(renderer): port the generic renderer, renderer thread and state

Ports renderer/{generic,cell,row,link,State,Overlay,image,Thread}.zig:
- generic.rs builds GPU cell buffers from terminal RenderState each
  frame: backgrounds, shaped text runs with glyph atlases and nerd font
  constraints, all cursor styles, decorations via sprites, selection and
  search colors, min-contrast, bold/faint/blink handling, preedit,
  padding extension, background image, custom shader uniforms, link
  hover underlines, scrollbar and health reporting.
- The renderer thread updates CPU-side state only and asks the host to
  redraw; the app runtime calls draw_frame from the GLArea render signal
  (the GL context lives on the GTK thread, as in Ghostty).
- ghostty_renderer::state::{State, SharedState} is the canonical shared
  renderer state (termio's interim copy will switch to it).

Deviations: overlay uses a direct RGBA rasterizer instead of z2d; bg
images decode with png/jpeg-decoder instead of wuffs; zero-length link
matches advance by one char (Zig would loop forever).

Kitty image rendering is implemented in image::State but not yet called
from generic::update_kitty (raced with the terminal-side wiring).

Test Plan:
- cargo test -p ghostty-renderer (69 unit + 9 headless EGL tests that
  render a Terminal through the full pipeline and check pixels)

crates/ghostty-renderer/src/lib.rs

@@ -1,22 +1,46 @@
-//! ghostty-renderer: OpenGL renderer building blocks (see
-//! docs/rewrite/ARCHITECTURE.md and docs/rewrite/notes/ghostty-renderer-R1.md).
+//! ghostty-renderer: the OpenGL renderer (see docs/rewrite/ARCHITECTURE.md and
+//! docs/rewrite/notes/ghostty-renderer-R1.md / -R2.md).
 //!
-//! Terminal-independent parts of `renderer/`: the OpenGL graphics API backend
-//! and its GPU object wrappers, shaders and their std140 data layouts,
-//! shadertoy custom-shader conversion, size math and cursor style logic.
+//! * R1: the OpenGL graphics API backend and its GPU object wrappers, shaders
+//!   and their std140 data layouts, shadertoy custom-shader conversion, size
+//!   math and cursor style logic.
+//! * R2: the generic renderer that builds GPU cells from a terminal
+//!   `RenderState` ([`generic`]), cell contents ([`cell`]), row heuristics,
+//!   link matching, kitty/overlay/background images ([`image`]), the debug
+//!   [`overlay`], the shared render [`state`] and the renderer [`thread`].
 
 #![cfg_attr(test, allow(clippy::unwrap_used))]
 
 pub mod backend;
+pub mod cell;
 pub mod cursor;
+pub mod generic;
+pub mod image;
+pub mod link;
 pub mod math;
 pub mod message;
 pub mod opengl;
 pub mod options;
+pub mod overlay;
+pub mod row;
 pub mod shadertoy;
 pub mod size;
+pub mod state;
+pub mod thread;
 
 pub use backend::{Backend, Health};
 pub use cursor::{Style as CursorStyle, style as cursor_style};
+pub use message::Message;
 pub use opengl::OpenGL;
 pub use size::{CellSize, Coordinate, GridSize, Padding, ScreenSize, Size};
+pub use state::State;
+
+/// The renderer used on Linux: the generic renderer instantiated with the
+/// OpenGL graphics API (`renderer.zig`: `pub const Renderer = GenericRenderer(OpenGL)`).
+pub type Renderer = generic::Renderer;
+
+/// The renderer's derived configuration (`Renderer.DerivedConfig`).
+pub type DerivedConfig = generic::DerivedConfig;
+
+/// The renderer thread (`renderer.Thread`).
+pub use thread::Thread;