feat(font): add the FreeType-only font backend (font-backend-freetype)

Ghostty can be built with -Dfont-backend=freetype, which uses FreeType
without any font discovery. The Rust port only had fontconfig+freetype
(default on Linux) and the CoreText variants. The new cargo feature
`font-backend-freetype` on ghostty-font (forwarded by ghostty-cli and
the root crate; non-Apple, non-wasm targets) behaves like Zig's mode:
font-family discovery, fallback search and codepoint-map overrides are
skipped (embedded fonts only); +list-fonts and +show-face print Zig's
"built without a font discovery mechanism" message and exit 1; +version
reports `.freetype`. The default backend is unchanged.

Also registers the Rust-only benchmark tooling extensions (--time,
--handler=full, ghostty-synthetic +sgr) in DEVIANT_FIXES.md.

Note: `--all-features` now selects this backend for the workspace.

Test Plan:
- cargo test -p ghostty-font --features font-backend-freetype (198)
- cargo build --features font-backend-freetype && ghostty +list-fonts

crates/ghostty-core/src/global.rs

@@ -146,9 +146,12 @@ pub fn default_crash_options() -> ghostty_crash::Options {
             .to_owned(),
         ..ghostty_crash::Options::default()
     };
+    ghostty_font::options
+        .backend
+        .name()
+        .clone_into(&mut o.font_backend);
     if cfg!(target_vendor = "apple") {
         "none".clone_into(&mut o.app_runtime);
-        "coretext".clone_into(&mut o.font_backend);
         "metal".clone_into(&mut o.renderer);
     }
     o