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-font/src/discovery.rs

@@ -10,6 +10,42 @@ pub use fc_impl::*;
 mod coretext_impl;
 #[cfg(font_coretext)]
 pub use coretext_impl::*;
+
+/// The `freetype` backend has no discovery mechanism: Zig's
+/// `Discover = void`. This is an uninhabited stand-in so that generic code
+/// type-checks; it can never be constructed, and the callers check
+/// [`crate::HAS_DISCOVERY`] (or `cfg(font_discovery)`) first.
+#[cfg(not(any(font_fontconfig, font_coretext)))]
+pub struct Discover(std::convert::Infallible);
+
+#[cfg(not(any(font_fontconfig, font_coretext)))]
+pub type DiscoverError = std::convert::Infallible;
+#[cfg(not(any(font_fontconfig, font_coretext)))]
+pub type NextError = std::convert::Infallible;
+
+#[cfg(not(any(font_fontconfig, font_coretext)))]
+pub struct DiscoverIterator(std::convert::Infallible);
+
+#[cfg(not(any(font_fontconfig, font_coretext)))]
+impl Discover {
+    #[must_use]
+    pub fn init() -> Discover {
+        unreachable!("no font discovery mechanism in this build")
+    }
+
+    pub fn discover(&self, _desc: &Descriptor) -> Result<DiscoverIterator, DiscoverError> {
+        match self.0 {}
+    }
+}
+
+#[cfg(not(any(font_fontconfig, font_coretext)))]
+impl DiscoverIterator {
+    #[allow(clippy::should_implement_trait)]
+    pub fn next(&mut self) -> Result<Option<crate::deferred_face::DeferredFace>, NextError> {
+        match self.0 {}
+    }
+}
+
 /// Descriptor is used to search for fonts (canonical definition in `ghostty-config`).
 pub use ghostty_config::font_types::Descriptor;