Integrate harfrust for text shaping (#8031)

* Related to #56 (Improve text — tracking issue)

## Summary

This PR integrates [harfrust](https://crates.io/crates/harfrust) (a
pure-Rust port of HarfBuzz) into epaint's text layout pipeline,
replacing the character-by-character glyph positioning with proper
OpenType text shaping.

### What this enables

- **GPOS kerning**: most modern fonts only ship kerning in GPOS tables
(not the legacy `kern` table). Pairs like "AV", "VA", "AT" are now
properly tightened.
- **GSUB substitutions**: ligatures (fi, fl), contextual alternates, and
other OpenType features.
- **Combining marks**: diacritics (e.g. ɔ̃) are positioned via anchor
tables instead of being rendered as standalone replacement glyphs.

### Before/After

#### Kerning, etc.

<img width="838" height="726" alt="before_main"
src="https://github.com/user-attachments/assets/f0f26d5f-b117-43a6-b39c-ea40d2e73836"
/>

<img width="838" height="726" alt="after_harfrust"
src="https://github.com/user-attachments/assets/d983e5da-486c-4f39-bd4f-5782a90c6b39"
/>

 #### Ligatures

<img width="1117" height="698" alt="before_closeup"
src="https://github.com/user-attachments/assets/7a3b08b4-cf6f-45b7-98ba-07c473cd3b02"
/>

<img width="1117" height="698" alt="after_closeup"
src="https://github.com/user-attachments/assets/6cfc5f21-d32f-4f09-be0c-59c8c553d44f"
/>

### Architecture

The shaping integrates into the existing pipeline without changing the
public API:

1. **`Font::segment_into_runs`** — segments text into contiguous runs by
font face (grapheme-cluster aware, never splits combining sequences)
2. **`FontFace::shape_text`** — calls harfrust to shape each run,
returning glyph IDs + positioned advances/offsets
3. **`layout_shaped_run`** — emits `Glyph` structs from the shaping
output, with NOTDEF fallback to other font faces for missing glyphs
4. **Buffer recycling** — `FontsImpl` pools a `harfrust::UnicodeBuffer`
to avoid per-layout allocations

### Disclaimer

I'm far from being a good Rust programmer. Claude Code did most of the
heavy lifting here. I did my best and used my limited knowledge to avoid
making too many mistakes. If this PR isn't up to quality standards,
please don't hesitate to close it.

## Test plan

- [x] `cargo test -p epaint` — all 18 text tests pass, including 6 new
ones
- [x] `cargo clippy -p epaint --all-features` — clean
- [x] `cargo fmt` — clean
- [ ] Snapshot tests need regeneration (expected: shaping changes glyph
positions)
- New tests added:
- `test_gpos_kerning` — verifies GPOS kerning tightens "AV", "VA", "AT"
pairs
- `test_combining_diacritics` — combining tilde doesn't add extra width
  - `test_shaping_basic_latin` — sanity check for Latin text
  - `test_shaping_empty_string` — empty input doesn't panic
  - `test_shaping_multiple_newlines` — newline splitting works correctly
  - `test_shaping_mixed_font_fallback` — Latin + emoji in same string

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

Cargo.lock

@@ -1540,6 +1540,7 @@ dependencies = [
  "emath",
  "epaint_default_fonts",
  "font-types",
+ "harfrust",
  "log",
  "mimalloc",
  "nohash-hasher",
@@ -1551,6 +1552,8 @@ dependencies = [
  "similar-asserts",
  "skrifa",
  "smallvec",
+ "unicode-general-category",
+ "unicode-segmentation",
  "vello_cpu",
 ]
 
@@ -1658,12 +1661,9 @@ dependencies = [
 
 [[package]]
 name = "fearless_simd"
-version = "0.3.0"
+version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8fb2907d1f08b2b316b9223ced5b0e89d87028ba8deae9764741dba8ff7f3903"
-dependencies = [
- "bytemuck",
-]
+checksum = "76258897e51fd156ee03b6246ea53f3e0eb395d0b327e9961c4fc4c8b2fa151a"
 
 [[package]]
 name = "file_dialog"
@@ -2018,6 +2018,15 @@ dependencies = [
  "bitflags 2.9.4",
 ]
 
+[[package]]
+name = "guillotiere"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6b17e70c989c36bad147b27a58d148c0741c51448aa5653436547323e524d0ab"
+dependencies = [
+ "euclid",
+]
+
 [[package]]
 name = "half"
 version = "2.6.0"
@@ -2029,6 +2038,19 @@ dependencies = [
  "num-traits",
 ]
 
+[[package]]
+name = "harfrust"
+version = "0.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9da2e5ae821f6e96664977bf974d6d6a2d6682f9ccee23e62ec1d134246845f9"
+dependencies = [
+ "bitflags 2.9.4",
+ "bytemuck",
+ "core_maths",
+ "read-fonts",
+ "smallvec",
+]
+
 [[package]]
 name = "hashbrown"
 version = "0.15.2"
@@ -3732,6 +3754,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7b634fabf032fab15307ffd272149b622260f55974d9fad689292a5d33df02e5"
 dependencies = [
  "bytemuck",
+ "core_maths",
  "font-types",
 ]
 
@@ -4732,6 +4755,12 @@ version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ce61d488bcdc9bc8b5d1772c404828b17fc481c0a582b5581e95fb233aef503e"
 
+[[package]]
+name = "unicode-general-category"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0b993bddc193ae5bd0d623b49ec06ac3e9312875fdae725a975c51db1cc1677f"
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.24"
@@ -4899,28 +4928,41 @@ dependencies = [
 ]
 
 [[package]]
-name = "vello_common"
-version = "0.0.6"
+name = "vello_api"
+version = "0.0.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1bd1a4c633ce09e7d713df1a6e036644a125e15e0c169cfb5180ddf5836ca04b"
+checksum = "a5088cd0113bc5332c753f24503825e3bc93e26c7883c9dc3ad9637bb62c4634"
+dependencies = [
+ "bytemuck",
+ "peniko",
+]
+
+[[package]]
+name = "vello_common"
+version = "0.0.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "986dc49a501a683477614bf07b8e7b6c79ae4828efce3bf22e51850f4a0a8a4c"
 dependencies = [
  "bytemuck",
  "fearless_simd",
+ "guillotiere",
  "hashbrown 0.16.1",
  "log",
  "peniko",
  "skrifa",
  "smallvec",
+ "thiserror 2.0.18",
 ]
 
 [[package]]
 name = "vello_cpu"
-version = "0.0.6"
+version = "0.0.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0162bfe48aabf6a9fdcd401b628c7d9f260c2cbabb343c70a65feba6f7849edc"
+checksum = "a678f91c7524a3a9ac9a19df9f83552866ec70b2ca26441b916a6b219b6aa2de"
 dependencies = [
  "bytemuck",
  "hashbrown 0.16.1",
+ "vello_api",
  "vello_common",
 ]