fix: Use fixed-point fractional x-advance and kerning for better text layout (#1168)

## Summary

**What is the goal of this PR?**

Hopefully fixes #1182.

_Note: I think letterforms got a "heavier" appearance after #1098, which
makes this more noticeable. The current version of this PR reverts the
change to add `--force-autohint` for Bookerly, which to me seems to
bring the font back to a more aesthetic and consistent weight._

#### Problem

Character spacing was uneven in certain words. The word "drew" in
Bookerly was the clearest example: a visible gap between `d` and `r`,
while `e` and `w` appeared tightly condensed. The root cause was
twofold:

1. **Integer-only glyph advances.** `advanceX` was stored as a `uint8_t`
of whole pixels, sourced from FreeType's hinted `advance.x` (which
grid-fits to integers). A glyph whose true advance is 15.56px was stored
as 16px -- an error of +0.44px per character that compounds across a
line.

2. **Floor-rounded kerning.** Kern adjustments were converted with
`math.floor()`, which systematically over-tightened negative kerns. A
kern of -0.3px became -1px -- a 0.7px over-correction that visibly
closed gaps.

Combined, these produced the classic symptom: some pairs too wide,
others too tight, with the imbalance varying per word.

#### Solution: fixed-point accumulation with 1/16-pixel resolution, for
sub-pixel precision during text layout

All font metrics now use a "fixed-point 4" format -- 4 fractional bits
giving 1/16-pixel (0.0625px) resolution. This is implemented with plain
integer arithmetic (shifts and adds), requiring no floating-point on the
ESP32.

**How it works:**

A value like 15.56px is stored as the integer `249`:

```
249 = 15 * 16 + 9    (where 9/16 = 0.5625, closest to 0.56)
```

Two storage widths share the same 4 fractional bits:

| Field | Type | Format | Range | Use |
|-------|------|--------|-------|-----|
| `advanceX` | `uint16_t` | 12.4 | 0 -- 4095.9375 px | Glyph advance
width |
| `kernMatrix` | `int8_t` | 4.4 | -8.0 -- +7.9375 px | Kerning
adjustment |

Because both have 4 fractional bits, they add directly into a single
`int32_t` accumulator during layout. The accumulator is only snapped to
the nearest whole pixel at the moment each glyph is rendered:

```cpp
int32_t xFP = fp4::fromPixel(startX);     // pixel to 12.4: startX << 4

for each character:
    xFP += kernFP;                          // add 4.4 kern (sign-extends into int32_t)
    int xPx = fp4::toPixel(xFP);           // snap to nearest pixel: (xFP + 8) >> 4
    render glyph at xPx;
    xFP += glyph->advanceX;                // add 12.4 advance
```

Fractional remainders carry forward indefinitely. Rounding errors stay
below +/- 0.5px and never compound.

#### Concrete example: "drew" in Bookerly

**Before** (integer advances, floor-rounded kerning):

| Char | Advance | Kern | Cursor | Snap | Gap from prev |
|------|---------|------|--------|------|---------------|
| d | 16 px | -- | 33 | 33 | -- |
| r | 12 px | 0 | 49 | 49 | ~2px |
| e | 13 px | -1 | 60 | 60 | ~0px |
| w | 22 px | -1 | 72 | 72 | ~0px |

The d-to-r gap was visibly wider than the tightly packed `rew`.

**After** (12.4 advances, 4.4 kerning, fractional accumulation):

| Char | Advance (FP) | Kern (FP) | Accumulator | Snap | Ink start | Gap
from prev |

|------|-------------|-----------|-------------|------|-----------|---------------|
| d | 249 (15.56px) | -- | 528 | 33 | 34 | -- |
| r | 184 (11.50px) | 0 | 777 | 49 | 49 | 0px |
| e | 208 (13.00px) | -8 (-0.50px) | 953 | 60 | 61 | 1px |
| w | 356 (22.25px) | -4 (-0.25px) | 1157 | 72 | 72 | 0px |

Spacing is now `0, 1, 0` pixels -- nearly uniform. Verified on-device:
all 5 copies of "drew" in the test EPUB produce identical spacing,
confirming zero accumulator drift.

#### Changes

**Font conversion (`fontconvert.py`)**
- Use `linearHoriAdvance` (FreeType 16.16, unhinted) instead of
`advance.x` (26.6, grid-fitted to integers) for glyph advances
- Encode kern values as 4.4 fixed-point with `round()` instead of
`floor()`
- Add `fp4_from_ft16_16()` and `fp4_from_design_units()` helper
functions
- Add module-level documentation of fixed-point conventions

**Font data structures (`EpdFontData.h`)**
- `EpdGlyph::advanceX`: `uint8_t` to `uint16_t` (no memory cost due to
existing struct padding)
- Add `fp4` namespace with `constexpr` helpers: `fromPixel()`,
`toPixel()`, `toFloat()`
- Document fixed-point conventions

**Font API (`EpdFont.h/cpp`, `EpdFontFamily.h/cpp`)**
- `getKerning()` return type: `int8_t` to `int` (to avoid truncation of
the 4.4 value)

**Rendering (`GfxRenderer.cpp`)**
- `drawText()`: replace integer cursor with `int32_t` fixed-point
accumulator
- `drawTextRotated90CW()`: same accumulator treatment for vertical
layout
- `getTextAdvanceX()`, `getSpaceWidth()`, `getSpaceKernAdjust()`,
`getKerning()`: convert from fixed-point to pixel at API boundary

**Regenerated all built-in font headers** with new 12.4 advances and 4.4
kern values.

#### Memory impact

Zero additional RAM. The `advanceX` field grew from `uint8_t` to
`uint16_t`, but the `EpdGlyph` struct already had 1 byte of padding at
that position, so the struct size is unchanged. The fixed-point
accumulator is a single `int32_t` on the stack.

#### Test plan

- [ ] Verify "drew" spacing in Bookerly at small, medium, and large
sizes
- [ ] Verify uppercase kerning pairs: AVERY, WAVE, VALUE
- [ ] Verify ligature words: coffee, waffle, office
- [ ] Verify all built-in fonts render correctly at each size
- [ ] Verify rotated text (progress bar percentage) renders correctly
- [ ] Verify combining marks (accented characters) still position
correctly
- [ ] Spot-check a full-length book for any layout regressions

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**YES, Claude Opus 4.6
helped figure out a non-floating point approach for sub-pixel error
accumulation**_

lib/EpdFont/EpdFont.h

@@ -12,7 +12,7 @@ class EpdFont {
 
   const EpdGlyph* getGlyph(uint32_t cp) const;
 
-  /// Returns the kerning adjustment (in pixels) between two codepoints.
+  /// Returns the kerning adjustment (4.4 fixed-point in pixels) between two codepoints.
   /// Returns 0 if no kerning data exists for the pair.
   int8_t getKerning(uint32_t leftCp, uint32_t rightCp) const;