Make font hinting target configurable via FontTweak (#8262)

Fixes #8079, where font hinting only sharpens the vertical axis
(vertical stems stay blurry), and none of the skrifa knobs were
reachable — only switching to `Target::Mono` helped, but that wasn't
exposed.

This makes the hinting target configurable instead of hardcoding
`Target::Smooth { symmetric_rendering: true, preserve_linear_metrics:
true }`.

### API
- New `epaint::text::HintingTarget` mirroring `skrifa::outline::Target`:
  - `Mono`
- `Smooth(SmoothHinting)` where `SmoothHinting { light,
symmetric_rendering, preserve_linear_metrics }`
- New field `FontTweak::hinting_target: HintingTarget`.
- Each variant/field is documented with what it does and the
egui-specific caveats (e.g. `symmetric_rendering` only affects
interpreter-hinted fonts; egui positions glyphs from shaper advances so
`preserve_linear_metrics` mostly affects sharpness, not layout).

### Render
- `font.rs` converts `HintingTarget` → `skrifa::outline::Target` and
threads it into the per-glyph `reconfigure` call; the hinting instance
is also reconfigured when the target changes.

### UI
- The font-tweak settings panel gets a `hinting_target` row: Smooth/Mono
radios, `light` / `symmetric_rendering` / `preserve_linear_metrics`
checkboxes, and a `Reset` button — all with tooltips.

### Behavior
- `HintingTarget::default()` matches egui's previous hardcoded target,
so **rendering is unchanged** unless you opt in. To fix the horizontal
blur from #8079, uncheck `preserve_linear_metrics` (or pick `Mono`).
Whether to flip the *default* is left as a follow-up.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

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

crates/epaint/src/text/mod.rs

@@ -10,7 +10,8 @@ mod text_layout_types;
 pub use {
     fonts::{
         FontData, FontDefinitions, FontFamily, FontId, FontInsert, FontPriority, FontTweak,
-        FontVariationAxis, Fonts, FontsImpl, FontsView, InsertFontFamily,
+        FontVariationAxis, Fonts, FontsImpl, FontsView, HintingTarget, InsertFontFamily,
+        SmoothHinting,
     },
     index::{ByteIndex, ByteRange, ByteRangeExt, CharIndex, CharRange, CharRangeExt},
     text_layout::*,