feat(docs-rs): Render knobs controls for playground code blocks in build pipeline

[ssilvius]

Goal

Wire up the full pipeline so that playground code blocks render both a preview (Web Component) and a <rafters-knobs> controls panel, while live blocks render preview-only (no knobs). Update the builder, template, and markdown rendering to support this.

Exact Implementation Requirements

Builder Changes (`builder.rs`)

• In `build_page()`, handle both `is_live()` and `is_playground()` code blocks
• For `live` blocks: existing behavior (preview + source code)
• For `playground` blocks: preview + knobs + source code
  • Use `generate_knobs_config()` from the transformed block
  • Render `` in the HTML
• Track whether any playground blocks exist on the page
• If any playground blocks exist, include `knobs.js` script in the page

Template Changes (`templates.rs`)

• Add `has_playground: bool` to the `Context` struct
• In `BASE_TEMPLATE`, conditionally include knobs JS:

  {% if has_playground %}<script src="{{ base_url }}assets/knobs.js"></script>{% endif %}

Asset Changes (`assets.rs`)

• In `generate_assets()`, write `knobs.js` to the assets directory

Markdown Rendering (`builder.rs` `render_markdown`)

• For playground blocks, insert preview container + knobs element + source code:

  <div class="preview-container" id="{block_id}">
    <{custom-element} ...>{content}</{custom-element}>
  </div>
  <rafters-knobs for="{block_id}" config='{knobs_config}'></rafters-knobs>

  Followed by the syntax-highlighted source code block

Behavior Requirements

• `live` blocks: unchanged behavior (preview + source)
• `playground` blocks: preview + knobs controls + source
• Pages with zero playground blocks: no `knobs.js` loaded
• The preview container gets an `id` attribute matching the `for` attribute on ``

Acceptance Criteria

Functional Tests Required

#[tokio::test]
async fn playground_block_includes_knobs() {
    // Create a temp site with a playground code block
    // Build it
    // Verify output HTML contains <rafters-knobs> element
    // Verify knobs.js is included
}

#[tokio::test]
async fn live_block_excludes_knobs() {
    // Create a temp site with a live code block (not playground)
    // Build it
    // Verify output HTML does NOT contain <rafters-knobs>
    // Verify knobs.js is NOT included
}

What NOT to Include

• The knobs Web Component itself (feat(docs-rs): Create reusable rafters-knobs Web Component for playground controls #769)
• AST extraction (feat(docs-rs): Create ast_extract module for shared AST extraction with oxc_parser #765)
• Generator changes (refactor(docs-rs): Make Web Component generator discovery-driven with knobs config output #767)

File Locations

• `crates/rafters-static/src/builder.rs` - pipeline integration
• `crates/rafters-static/src/templates.rs` - add `has_playground` to Context
• `crates/rafters-static/src/assets.rs` - write knobs.js asset

Integration Requirements

Dependencies

• refactor(docs-rs): Make Web Component generator discovery-driven with knobs config output #767: `TransformedBlock.knobs_config` field
• refactor(docs-rs): Rename BlockMode::Editable to Playground #768: `BlockMode::Playground` and `is_playground()` method
• feat(docs-rs): Create reusable rafters-knobs Web Component for playground controls #769: `AssetPipeline::generate_knobs_js()`

Success Criteria

• Playground blocks render preview + knobs + source
• Live blocks render preview + source (no knobs)
• Source-only blocks render just source (no change)
• `knobs.js` only loaded when playground blocks exist
• `cargo test --workspace` passes
• `cargo clippy --workspace --all-targets` clean

This issue is complete when: A page with ```````tsx playground``````` renders an interactive preview with variant/size/boolean knobs, while ```````tsx live``````` renders preview-only.

Context & References

• Depends on: refactor(docs-rs): Make Web Component generator discovery-driven with knobs config output #767, refactor(docs-rs): Rename BlockMode::Editable to Playground #768, feat(docs-rs): Create reusable rafters-knobs Web Component for playground controls #769
• Current build pipeline: `builder.rs:380-456`
• Current markdown rendering: `builder.rs:534-590`