feat: Support for proportional numeral spacing (#1414)

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## Summary

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

Reading a book with frequent numbers, I noticed that the spacing between
numeral glyphs was strangely large. This was because Bookerly and Noto
Sans default to tabular figures, where every digit gets an identical
advance width. This is designed for column alignment in spreadsheets,
but in rendering prose it produces visually wide gaps between digits.

This change adds a `--pnum` flag to fontconvert.py that applies the
font's OpenType `pnum` (proportional numerals) feature during
conversion. When active, the converter:
- Parses the GSUB table for pnum SingleSubst lookups
- Resolves substitute glyph indices via fonttools' glyph order
- Loads the proportional alternate glyphs instead of the tabular
defaults
- Includes substitute glyph names in kern pair extraction, so kerning
data that references proportional alternates is captured

Bookerly's proportional alternates also carry digit-digit and
digit-punctuation kerning that the tabular glyphs lack (e.g., at 16pt
7->4 at -1.69px, 7->. at -2.31px, 7->1 at +1.00px).

Noto Sans gains proportional advances but no new kerning (its
proportional glyphs have no kern class data in the font).

OpenDyslexic is unaffected. Its `cmap` already points to proportional
glyphs, so `--pnum` is a no-op. `--pnum` is intentionally omitted from
OpenDyslexic in the build script for deliberately uniform digit spacing
as an accessibility choice.

UI fonts (Ubuntu, notosans_8) also omit `--pnum` to preserve tabular
alignment for page numbers, battery percentages, etc.

| Before | After |
| -- | -- |
| <img
src="https://github.com/user-attachments/files/26042238/screenshot-31673.bmp"
width="300" /> | <img
src="https://github.com/user-attachments/files/26042241/screenshot-124075.bmp"
width="300" /> |

---

### 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**_

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