Handle relative links in LiaScript YAML frontmatter; remove invalid @import pattern

[Copilot]

The packer missed assets declared in the LiaScript YAML header (e.g. import, link, script, logo) and had a spurious @import regex pattern that doesn't correspond to any real LiaScript syntax.

Changes

• Removed the @import regex from _LINK_PATTERNS — not a LiaScript feature
• Added _extract_yaml_frontmatter_links() — lightweight line-by-line parser for the YAML block between --- markers; extracts file references from import, link, script, and logo fields, handling both scalar and list values, quoted or unquoted
• Integrated YAML extraction into extract_relative_links() before the body regex pass, with shared dedup/absolute-URL filtering

Example

A header like:

---
title: My Course
logo: images/banner.jpg
link:
  - styles/custom.css
import:
  - macros/helpers.md
  - https://cdn.example.com/remote.md   # skipped — absolute
script: js/init.js
---

now correctly adds images/banner.jpg, styles/custom.css, macros/helpers.md, and js/init.js to the ZIP, while ignoring the absolute URL.

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