docs: add README, changelog entry and top-down architecture overview

README explains what the rewrite covers, how to build, run, install and
test it. ARCHITECTURE.md gains a top-down decomposition tree (binary ->
runtime/core -> termio/terminal/renderer -> leaf crates) and an updated
crate map with the actual internal dependencies, including the crates
added during integration (apprt, inspector, onig, bench, build-data).
CHANGELOG records the rewrite under Unreleased; no version bump.

CHANGELOG.md

@@ -5,3 +5,22 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+- Complete Rust rewrite of Ghostty 1.3.1 for Linux: VT parser and stream,
+  terminal core (page list, screen, terminal, selection, formatter, search,
+  kitty graphics, tmux control mode), font engine (fontconfig, freetype,
+  harfbuzz, sprite font), OpenGL renderer with custom shader support,
+  termio (pty, child process, shell integration, terminfo), configuration
+  system with all options and docs, input and keybindings, core App/Surface,
+  GTK4/libadwaita app runtime, terminal inspector, and all `ghostty +action`
+  CLI commands.
+- Bundled resources: shell integration, 463 themes, compiled terminfo,
+  desktop integration files, translations.
+- `ghostty-bench` (benchmarks) and `ghostty-build-data` (shell completions,
+  Vim/Sublime/bat syntax, man pages).
+- About 3000 tests, including all ported Zig tests and headless end-to-end
+  tests of the core with a real pty.