On Windows, path.relative() and path.join() return backslash separators.
Graph node keys were stored with native separators, but query inputs use
forward slashes, causing silent lookup failures on Windows.
Add toForwardSlash() utility and apply it at build time (file walker,
resolution functions) and query time (getFileDependencies,
getSymbolContext, listSymbols) for defense-in-depth.
No-op on macOS/Linux where path.relative() already returns forward
slashes. Existing Windows symbol graph caches require one rebuild.
Fixes#60
Giancarlo Erra