Replace front matter with @slide directive for slide configuration

## Summary

Propose replacing YAML front matter (`--- ... ---`) with a `@slide` directive for configuring slide-level options inline, consistent with how all other SuperDeck annotations work.

## Current behavior

Today, slide configuration uses YAML front matter between `---` delimiters at the top of each slide:

```markdown
---
title: Welcome
style: announcement
template: corporate
owner: Platform Team
---

@block
# Hello World
```

This is parsed by `FrontmatterParser` which extracts the YAML block, converts it to a `Map`, and feeds it into `SlideOptions.parse(...)`.

### Known `SlideOptions` fields

| Field | Type | Purpose |
|-------|------|---------|
| `title` | `String?` | Slide title (navigation, export) |
| `style` | `String?` | Named style variant from `DeckOptions.styles` |
| `template` | `String?` | Named template from `DeckOptions.templates` (`'none'` to opt out) |
| `args` (catch-all) | `Map` | Any extra keys, accessible via `slide.options.args['key']` |

### How it flows through the system

1. `MarkdownParser` splits the markdown file into slides (splitting on `---`)
2. `FrontmatterParser` extracts front matter YAML from each slide
3. `SlideOptions.parse(frontmatter)` validates and creates options
4. `SectionParser` parses the remaining content into `@section`/`@block`/`@widget` layout
5. `CommentParser` extracts HTML comments (``) as speaker notes
6. Everything compiles into a `Slide` object with `key`, `options`, `sections`, `comments`

## Problem

The `---` delimiter serves a dual purpose: it both **separates slides** and **opens/closes front matter**. This creates ambiguity:
- A `---` in the middle of a slide could be mistaken for a slide separator
- The front matter syntax feels disconnected from the `@directive { options }` pattern used everywhere else

## Proposed solution: `@slide` directive

Replace front matter with `@slide`:

```markdown
@slide {
title: Welcome
style: announcement
template: corporate
owner: Platform Team
}

@block
# Hello World
```

### Benefits
- Consistent with `@section`, `@block`, `@widget` syntax
- Uses the same YAML options parsing (`TagTokenizer` already handles `@slide { ... }`)
- Clearer separation of concerns: `---` only separates slides, `@slide` only configures them
- More discoverable for new users (everything is an `@` directive)

### Slide separation
Slides would still be separated by `---`. The `@slide` directive would be optional (slides without it just have no options).

### Backwards compatibility
- YAML front matter could be supported alongside `@slide` during a transition period
- If both are present, `@slide` takes precedence (or it's an error)

## Acceptance criteria

- [ ] `@slide { ... }` is parsed and populates `SlideOptions` (title, style, template, args)
- [ ] `---` continues to work as a slide separator only
- [ ] Slides without `@slide` work normally (no options)
- [ ] Legacy front matter still works (deprecation warning)
- [ ] `TagTokenizer` already supports `@slide` tokenization (no parser changes needed)
- [ ] Documentation updated

## Related

- `packages/builder/lib/src/parsers/front_matter_parser.dart` — current front matter extraction
- `packages/builder/lib/src/parsers/block_parser.dart` — tag-based parsing (would need to handle `@slide` specially)
- `packages/core/lib/src/deck/slide_model.dart` — `SlideOptions` model
- `packages/core/lib/src/markdown/tag_tokenizer.dart` — already parses `@name { yaml }` tokens