Improve config layering and HTML help navigation

[fasterthanlime]

Summary

This PR tightens Figue's config layering around multiple config roots, CLI-provided config files, boolean config overrides, and flattened config roots. It also includes the HTML help feedback pass so generated help remains navigable and readable for large schemas.

Changes

• Target CLI-provided config files to the matching config root, and require root blocks for global config files when a command has multiple config roots.
• Allow bare boolean config overrides, including --flag, --flag=false, and --no-flag forms where applicable.
• Support #[facet(args::config)] #[facet(flatten)] roots so ergonomic CLI flags can still merge with env/file/dotted overrides under the config-root namespace, with collision checks.
• Preserve explicit defaults during schema/default extraction, including #[facet(default)] Vec<_> fields inside flattened config roots.
• Improve HTML help layout: scrollable side nav, wrapping long config names/descriptions, compact override rows without redundant value placeholders, and cleaner false/default display.
• Add sticky HTML help breadcrumbs that track the current section/config node and let users jump back to any ancestor.
• Treat flattened config fields as help-only collapsible groups using the flattened field docs, without changing actual override paths.

Testing

• cargo nextest run -p figue -E 'test(html_help)'
• cargo nextest run -p figue -E 'test(flattened_config_root)'
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo nextest run -p figue
• Browser smoke check against generated kitchen-sink HTML at 1141x1285 for breadcrumbs, sticky topbar, scrolling nav, wrapping, and horizontal overflow
• Pre-push checks passed