fix(filesystem): case-insensitive Windows allowed-path matching (#470)

[adityachaudhary99]

Closes #470

On Windows, containment checks between allowed roots and requested paths were case-sensitive, so an allowed root like c:\source rejected requests to C:\source\file.md (drive-letter and component-case mismatches).

• Canonicalize each allowed directory once at startup (path.resolve + realpathSync.native)
• Case-fold both sides during prefix comparison on win32 only; POSIX behavior byte-exact
• Extracted the decision into a pure isPathWithin() helper with segment-boundary semantics
• New unit tests: drive-letter case mismatch, inner-component mismatch, traversal escapes still denied, POSIX parity

Tested locally on Windows 11: filesystem suite 92/92 green.

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