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

Cargo.toml

@@ -14,6 +14,9 @@ path = "src/main.rs"
 sentry = ["ghostty-crash/sentry"]
 # Mirrors upstream's `-Dsnap` build option (off by default).
 snap = ["ghostty-os/snap"]
+# Mirrors upstream's `-Dfont-backend=freetype`: FreeType rendering with no
+# font discovery (fontconfig is not used). Non-Apple, non-wasm targets only.
+font-backend-freetype = ["ghostty-cli/font-backend-freetype"]
 
 [dependencies]
 ghostty-core.workspace = true