Integrate harfrust for OpenType text shaping in epaint

Replace the character-by-character glyph lookup in `layout_section()`
with a proper text shaping pipeline using harfrust (a Rust port of
HarfBuzz). This enables GPOS kerning, ligatures (fi, fl), and correct
positioning of combining diacritical marks.

The shaping pipeline works as follows:
1. Split text into font-fallback runs (grapheme-cluster-aware)
2. Shape each run with harfrust (GSUB + GPOS)
3. Allocate and position glyphs from the shaping output

Key changes:
- Add harfrust, unicode-segmentation, unicode-general-category deps
- Cache ShaperData on FontFace (parsed GSUB/GPOS tables)
- Add shape_text() with buffer flags and variable font support
- Add allocate_glyph_by_id() for shaper-produced glyph IDs
- Recycle harfrust UnicodeBuffer across layout calls
- Handle NOTDEF fallback (combining marks via unicode-general-category)

Addresses #2517.

crates/epaint/Cargo.toml

@@ -63,6 +63,7 @@ ecolor.workspace = true
 
 ahash.workspace = true
 font-types.workspace = true
+harfrust.workspace = true
 log.workspace = true
 nohash-hasher.workspace = true
 parking_lot.workspace = true # Using parking_lot over std::sync::Mutex gives 50% speedups in some real-world scenarios.
@@ -70,6 +71,8 @@ profiling.workspace = true
 self_cell.workspace = true
 skrifa.workspace = true
 smallvec.workspace = true
+unicode-general-category.workspace = true
+unicode-segmentation.workspace = true
 vello_cpu.workspace = true
 
 #! ### Optional dependencies