[api] Gate the experimental shape() free function (#406)

The `harfrust::shape` free function is re-exported only under the
`experimental_font_api` feature (it takes a `FontInstance`, which is
public only under that feature), but its definition was not gated to
match.  In a default-feature build the re-export is compiled out while
the function remains, leaving it unreachable and triggering a dead_code
warning (visible during `cargo publish` verification).

Gate the definition with the same `#[cfg(feature =
"experimental_font_api")]`.  No functional change; the experimental API
is unaffected and there is no cascade -- its helpers stay reachable
through other paths.

Assisted-by: Claude <noreply@anthropic.com>

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: behdad

harfrust/src/hb/face.rs

@@ -436,6 +436,7 @@ impl Scale {
 ///
 /// Will panic when debugging assertions are enabled if the buffer and plan have mismatched
 /// properties.
+#[cfg(feature = "experimental_font_api")]
 pub fn shape(
     font: &crate::font::FontInstance,
     mut buffer: UnicodeBuffer,