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-cli/src/show_face.rs

@@ -86,6 +86,16 @@ cli_options!(Options, help, diagnostics);
 ///     style. Valid options are `text` and `emoji`. If unset, the presentation
 ///     style of a codepoint will be inferred from the Unicode standard.
 pub fn run(args: &[impl AsRef<[u8]>]) -> Result<u8, Error> {
+    // Its possible to build Ghostty without font discovery! (This is checked
+    // before the arguments are parsed, as in Zig; the stderr writer is on
+    // stdout, like the other show-face writers.)
+    if !ghostty_font::HAS_DISCOVERY {
+        let mut stderr = ZigWriter::new(stdout_sink(), 1024);
+        write!(stderr, "{}", crate::list_fonts::NO_DISCOVERY_MESSAGE)?;
+        stderr.flush()?;
+        return Ok(1);
+    }
+
     let mut opts = Options::default();
     crate::parse_options(&mut opts, args)?;