fix: stabilize reader and font generation

[SamLaio]

Summary / 摘要

Type of change / 改動類型

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation only / 純文件
• Refactor / 重構（不改變功能）
• Build / tooling / config

Stage label / Stage 標籤

Proposed stage:

Affected branches / 影響的分支

• stage7/epub-fast-open (繁中 main)
• stage5.5/jianti (簡中 main)
• stage12.5/* (current recommended)
• stage11/jianti-with-jianguo (stageA, friendly fork)
• Other:

Verified on hardware / 已實機驗證

• Built successfully (繁中) — Flash usage: __%
• Built successfully (簡中) — Flash usage: __%
• Flashed to physical X4
• Bluetooth page-turner test (which device?):
• Large EPUB test (which book?):
• No regression on previously-working features

Code checklist / 程式碼檢查

• No hardcoded local paths (no C:\Users\...)
• No API keys, tokens, or credentials
• No personally-identifying info beyond the author byline
• Comments where the why is non-obvious (not commenting the what)
• If touching BluetoothHIDManager, tested re-pair flow with a real device
• If touching lib/Epub/, tested with both small (<1MB) and large (>10MB) EPUBs
• If touching lib/EpdFont/, regenerated affected .h files via fontconvert.py

Notes / 備註