Add wikilink rendering extension

[soreavis]

Adds support for [[wikilinks]] (closes #63). cmark-gfm reserves [ for its own link parser, so this is a post-process cmark-extra extension (wikilink.c) modeled on cmark-gfm's own autolink — it splits text nodes after parsing, leaving the cmark engine untouched.

[[Page Name]] and [[Target|Display]] are rewritten into styled links:

See [[Getting Started]] and the [[Reference|API reference]].

→

<p>See <a class="wikilink" href="Getting%20Started">Getting Started</a> and the <a class="wikilink" href="Reference">API reference</a>.</p>

Behaviour

• Off by default. The inner text must be non-empty, alone between the brackets, and free of [/] — so ordinary text, malformed/nested brackets, code spans, and existing Markdown links are left untouched.
• [[Target|Display]] splits on the first | (target → href, the rest → label); [[Target|]] falls back to the target as the label.
• Leading/trailing whitespace in the target and label is trimmed.
• Security: the target is houdini-escaped into the href and dangerous URLs (javascript: …) are dropped in safe mode — identical to how cmark renders native links; labels are HTML-escaped. (Validated under AddressSanitizer + leaks on adversarial input: no over-reads, use-after-free, or leaks.)
• default.css styles .wikilink (light #8250df / dark #ab7df8), distinct from regular links.

How it's exposed

• Preferences UI — a new "Wikilinks" checkbox in the Extensions grid, wired like the other extension toggles.
• CLI — a --wikilink on|off option.
• Settings — a wikilinkExtension flag (Codable + app-group defaults), defaulting to off.

Examples

• Adds examples/test-wikilink.md (basic, aliased, paths, multiple-per-line, plus the edge cases: empty/whitespace markers, malformed brackets, code spans, Markdown-inside).
• Enriches examples/test1.md — the file shown in the Settings live preview — with a "Wikilink extension" section and its TOC entry.

Changed files

• cmark-extra/extensions/wikilink.{c,h}, extra-extensions.c — the extension and its registration
• QLMarkdown/Settings.swift, Settings+render.swift, Settings+ext.swift — the flag and render wiring
• qlmarkdown_cli/qlmarkdown_cli.swift — the CLI option
• QLMarkdown/ViewController.swift, Base.lproj/Main.storyboard — the Preferences checkbox
• Resources/default.css — the .wikilink style (light & dark)
• examples/test-wikilink.md, examples/test1.md — examples

Closes #63.