test: expand CodeFragment coverage and docs

[adamcavendish]

Summary

• Add broader CodeFragment coverage for imports, nested code block args, template composition, and unbalanced indentation marker validation.
• Add a runnable code_fragments example showing Python indentation composition through (CodeFragment).
• Document raw %L vs structured CodeFragment behavior, including UnresolvedIndentMarker and balanced %> / %< requirements.

Tests

• cargo test -p sigil-stitch fragment -- --nocapture
• cargo test --test cpp
• cargo test
• cargo run --example code_fragments
• git diff --check

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 98.64865% with 1 line in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/code_block.rs	98.64%	1 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Files with missing lines	Coverage Δ
src/code_block.rs	92.35% <98.64%> (+0.79%)	⬆️

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.