Make preview source matching path-safe across React fallback and definition lookup

[rioredwards]

Preview source matching still relies on string-prefix and substring checks, which can resolve the wrong file when repos share path prefixes or when separators differ across platforms.

Problem

• src/reactFiberAdapter.ts uses substring-style matching for source file comparisons
• src/hoverProvider.ts uses fsPath.startsWith(workspaceRoot) when filtering definition results
• sibling repos such as /repo and /repo-other can be treated as the same workspace prefix
• node_modules filtering only checks /node_modules/, which misses Windows-style paths

Suggested fix

• Replace startsWith and substring checks with normalized, segment-aware path comparisons based on path.relative
• Normalize separators before applying node_modules filtering
• Add tests for /repo vs /repo-other, Windows-style paths, and same-basename files in different folders

Relevant files

• src/reactFiberAdapter.ts
• src/hoverProvider.ts
• src/pathUtils.ts