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/shaper/mod.rs

@@ -19,5 +19,5 @@ pub mod run;
 
 pub mod web_canvas;
 
-#[cfg(all(test, font_harfbuzz, font_fontconfig))]
+#[cfg(all(test, font_harfbuzz))]
 pub(crate) mod test_util;

crates/ghostty-font/src/shaper/test_util.rs

@@ -23,7 +23,6 @@ use crate::{
     Style,
     codepoint_resolver::CodepointResolver,
     collection::{AddOptions, Collection, LoadOptions, SizeAdjustment},
-    discovery::{Descriptor, Fontconfig},
     embedded,
     face::{DesiredSize, FontData, Options as FaceOptions},
     shape,
@@ -102,7 +101,18 @@ pub(crate) fn test_shaper_with_font(font_req: TestFont) -> TestShaper {
 
 /// Returns None (the test is skipped, like `error.SkipZigTest`) if the font
 /// can't be discovered.
+#[cfg(not(font_fontconfig))]
+pub(crate) fn test_shaper_with_discovered_font(_font_req: &str) -> Option<TestShaper> {
+    // No fontconfig discovery in this backend (for example `freetype`).
+    None
+}
+
+/// Returns None (the test is skipped, like `error.SkipZigTest`) if the font
+/// can't be discovered.
+#[cfg(font_fontconfig)]
 pub(crate) fn test_shaper_with_discovered_font(font_req: &str) -> Option<TestShaper> {
+    use crate::discovery::{Descriptor, Fontconfig};
+
     let lib = Library::init().unwrap();
     let mut c = Collection::new();
     c.load_options = Some(LoadOptions::new(lib.clone()));