🎨 Implement syntax highlight pre-processing transform

[Copilot]

Adds a SyntaxHighlightTransform that normalizes fenced code block language tags before rendering, ensuring consistent downstream processing by Pandoc.

Changes

• src/transforms/syntax_highlight.rs — New transform that detects ``` fenced code blocks and normalizes language tags to lowercase with whitespace trimmed; code body is passed through unchanged
• src/transforms/registry.rs — Registers SyntaxHighlightTransform as the 3rd step in the standard pipeline (after emoji → variables → syntax)
• src/transforms/mod.rs — Exports SyntaxHighlightTransform from the transforms module

Behavior

<!-- Input -->
```  Python  
print("hi")
` `` `

<!-- Output -->
```python
print("hi")
` `` `

Edge cases handled: unclosed blocks, nested backtick lines inside a block, missing language tag, trailing newline preservation.

Tests

14 unit tests in syntax_highlight.rs + 4 registry-level integration tests covering the full transform chain.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>🎨 Implement syntax highlight pre-processing transform</issue_title>
<issue_description>### Title: 🎨 Implement syntax highlight pre-processing transform

---

Description

Add a transform to prepare code blocks for syntax highlighting.

This prepares content before rendering to HTML/PDF.

---

Goals

• Improve code block rendering
• Prepare for advanced formatting
• Enable future styling control

---

Requirements

---

1. Detect Code Blocks

Identify:

```language
code

---

## 2. Transform Behavior

Options:

- inject classes for styling
- normalize language tags
- prepare for Pandoc highlighting

---

## 3. Keep It Simple

V1:

- detect code blocks
- ensure language tags are preserved

---

### Tests

- code blocks remain valid
- transform does not break content

---

### Acceptance Criteria

- [ ] Transform detects code blocks
- [ ] Content remains valid Markdown
- [ ] Integrated into pipeline
- [ ] Tests pass

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 🎨 Implement syntax highlight pre-processing transform #86

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