flattenTree()function

Flatten a tree into a Map for O(1) lookup, stamping a canonical path onto every element as it walks.

• Returns a copy of the tree, indexed. Each element is rebuilt with its canonical site-root-relative path: the root is /, each descendant is parentPath + "/" + name — so a module schema → /schema, its class BooleanSchema → /schema/BooleanSchema, its member validate → /schema/BooleanSchema/validate. A composite module name (e.g. "util/string") becomes its own multi-segment chunk.
• Every element is registered under two keys, both pointing at the (stamped) element:
  
  • its flat key — bare name (e.g. "BooleanSchema"), or qualified "Class.member" for members (e.g. "BooleanSchema.validate"). This is what cross-refs (extends / implements) and README links resolve through.
    
    • its canonical path (element.props.path, e.g. "/schema/BooleanSchema") — what the router resolves a URL to.
• The map values keep their (stamped) children, so the map doubles as the navigable tree: map.get("/") is the stamped root and flattening never throws away the hierarchy. The router resolves a URL with map.get(path); the static builder enumerates pages from the path-shaped keys.
• Each stamped element's own key is also set to its canonical path — so a flat (cross-tree) listing of stamped elements has globally-unique React keys, where the bare name would collide (many get / value / url).
• Exported names are unique across the package (barrel re-exports enforce it at compile time), so flat-key collisions are vanishingly rare; on a collision the last writer simply wins.
• Missing keys (e.g. builtins like Serializable) resolve to undefined → callers fall back to plain text.