Implement on-demand source processing

[Vladyslav-Kuksiuk]

[WIP]

This PR changes the source-processing flow from eager preprocessing to on-demand source resolution.

Previous flow:

1. All source code files were scanned up front.
2. Each source file was split into fragments, cleaned from fragment marker comments, normalized, and written into build/fragments.
3. Target documents were scanned and embedded snippets were read from those prepared fragment files.

New flow:

1. Target documents are scanned first.
2. Source files are processed only when they are referenced by an <embed-code> instruction.
3. Named fragments are resolved directly from source and cached in memory per source file.

Additional cleanup:

• Removed code-includes / code-excludes configuration, as it doesn't make sense.