Add @notes directive for slide speaker notes

[tilucasoli]

Summary

Introduce a first-class Markdown directive for speaker notes (e.g. @notes or @comment) that compiles into slide.comments.

Today, any @something token is treated as a tag, and unknown tags can look like they should work as “special blocks”, but they don’t produce speaker notes.

Motivation

• Authors intuitively try @comment / @notes for presenter notes.
• Speaker notes already exist in the compiled contract (slide.comments: string[]).
• A directive would make notes discoverable and keep them close to slide content.

Proposed solution

Option A (preferred): @notes (no options)

@notes
This is a presenter note.
- Not visible on the slide
- Exported into `slide.comments`

Option B: @comment

Same semantics as @notes.

Multi-note behavior

• Allow multiple @notes blocks per slide.
• Each block appends one entry to slide.comments.

Backwards compatibility

• @comment is currently parsed as a generic tag and could already be used as a widget shorthand if a deck registers a widget named comment. Making it reserved may be breaking.
• @notes is less likely to collide with existing widget names.

Acceptance criteria

• @notes content is captured as speaker notes and compiled into slide.comments.
• Notes do not render as visible slide content.
• Works with multiple @notes blocks in a slide.
• Documentation updated: docs/reference/markdown-syntax.mdx, docs/reference/block-types.mdx, and authoring guide.

Related

• docs/reference/contracts.mdx documents comments: string[].
• packages/core/lib/src/markdown/tag_tokenizer.dart currently treats any @name as a tag (outside fences).