Add ID-based element referencing in filesystem paths

[Copilot]

Elements can now be referenced by their id attribute in filesystem paths instead of only by numeric index.

Changes

Client path resolution (fusedom-client.js):

• buildDOMTree(): Uses {id}.{tagname} format when element has an id attribute, falls back to {index}.{tagname} otherwise
• findElementByPath(): Parses selectors as either numeric indices or ID strings, filtering by tag name in both cases

Documentation: Updated README.md, QUICKSTART.md, and index.html with examples of ID-based paths

Usage

# Before: numeric index only
echo "content" > ./mnt/0.html/0.body/0.div/innerHTML

# After: ID-based reference (when element has id="header")
echo "content" > ./mnt/0.html/0.body/header.div/innerHTML

# Backward compatible: numeric indices still work
echo "content" > ./mnt/0.html/0.body/0.div/innerHTML

Elements without IDs continue to use numeric indices. No breaking changes.

Original prompt

add referencing children by id (e.g. ./main.div/ instead of ./0.div)

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