fix: only extract properties from leading front-matter block

[dpecos]

Summary

• Fixes extractProperties in parser.ts to stop consuming key: value lines once a blank line or non-property line is encountered
• HTML comment properties (<!-- key: value -->) are still extracted anywhere in the source since they are invisible markers by design
• Updates the one existing test whose expectation was an artifact of the old regex leaving a leading newline
• Adds four new tests covering the bug scenario and boundary cases

Root cause

The old implementation used a single greedy regex with no anchor:

/^\n*([-\w]+):([^$\n]*)|\n*(?:<!--\s*)([-\w]+):([^$\n]*?)(?:\s*-->)/i

This matched any word: value line anywhere in the slide source, silently consuming content like Example: this line disappears as a property.

Fix

Property extraction now works like YAML front-matter:

1. Skip any leading blank lines
2. Consume consecutive key: value lines
3. Stop immediately at the first blank line or non-property line

Lines appearing after content are never treated as properties.

Closes #4