feat(diff): implemented native UTF-16 diff processing and removed jsdiff

- Implemented native UTF-16 text processing in Rust diff module with support for unpaired surrogates.
- Removed `similar` crate from Rust workspace and `diff` npm package from coding-agent, hashline, and natives.
- Removed jsdiff fallback wrappers and `isWellFormed()` guards from TypeScript diff implementations.
- Added comprehensive test suite for native diff functions covering random inputs and edge cases including surrogates and emoji.
- Renamed model `codex-auto-review` to `gpt-5.3-codex-spark` with updated pricing and context window.

packages/coding-agent/package.json

@@ -79,7 +79,6 @@
     "@xterm/headless": "catalog:",
     "arktype": "catalog:",
     "chalk": "catalog:",
-    "diff": "catalog:",
     "fast-xml-parser": "catalog:",
     "handlebars": "catalog:",
     "header-generator": "catalog:",