📝 Scribe: Add pen drawing example and doc comments

[khangnghiem]

What:

• Added a new example file examples/pen_drawing.fd demonstrating freehand drawing and Catmull-Rom curves.
• Added missing documentation (///) to public items in crates/fd-core/src/completion.rs and crates/fd-wasm/src/svg.rs.

Why:

• To fulfill the Scribe agent directive of picking an underused feature and creating a showcase .fd file.
• To improve repository health and developer experience by ensuring public Rust APIs are well-documented.

Impact:

• Provides an easy reference for how to create and style paths in Fast Draft documents.
• Makes the code completion and SVG export APIs easier to understand for future developers.

Measurement:

• The .fd document parses correctly using fdraft check.
• All cargo test suites continue to pass successfully.

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PR created automatically by Jules for task 17526938477256341644 started by @khangnghiem

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