Convert Godot BBCode cross-refs and codeblocks to Markdown

[deevus]

Summary

• Convert Godot-specific BBCode tags to proper Markdown using bbcodez's callback system
• Cross-reference tags ([member], [method], [param], [constant], [signal], [enum], [annotation], [theme_item]) render as inline code or italic
• Bare class references ([Node2D], [@GDScript]) render as inline code
• [codeblock] renders as fenced code blocks with auto-dedented content
• [codeblocks] with [gdscript]/[csharp] sections render as separate language-tagged fenced blocks

Before

See also [member position]. This [method rotate] uses [param delta].
[codeblock]
				print("hello")
[/codeblock]

After

See also `position`. This `rotate()` uses *delta*.

\`\`\`
print("hello")
\`\`\`

Test plan

• zig build test — all 14 new tests + existing tests pass
• gdoc Node2D — cross-refs converted in class index
• gdoc Node2D.look_at — [param], [method] converted in member docs
• gdoc Vector2.from_angle — [codeblock] renders as fenced code
• gdoc Rect2.grow — [codeblocks] renders both gdscript and csharp blocks
• gdoc CharacterBody2D.get_slide_collision — indented code preserves relative indentation