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/Cargo.toml

@@ -58,6 +58,11 @@ coretext-harfbuzz = ["dep:harfbuzz-sys"]
 # CoreText for discovery and rendering, no shaping
 # (Zig `-Dfont-backend=coretext_noshape`; Apple targets only).
 coretext-noshape = []
+# FreeType for rendering and HarfBuzz for shaping with no font discovery
+# (Zig `-Dfont-backend=freetype`; non-Apple, non-wasm targets only; the
+# default there is `fontconfig_freetype`). Not enabled by `--all-features`
+# in any meaningful sense: it switches the whole build to the backend.
+font-backend-freetype = []
 
 # This crate wraps the freetype / harfbuzz / fontconfig C libraries through
 # their `-sys` crates; unsafe is denied by default and allowed on minimal,