Do not report every existing Windows path as accessible for every mode

[LunaStev]

The Windows access(path, mode) implementation ignores mode and only checks whether GetFileAttributes succeeds. Consequently R_OK, W_OK, X_OK and invalid mode bits are all treated like F_OK. Callers asking whether an operation is permitted can receive success solely because the path exists.

Source evidence at the head of #520:

• std/sys/windows/fs.wave:188 — pub fun access
• std/sys/windows/fs.wave:52 — pub const FS_X_OK

Acceptance:

• Validate mode bits and retain a distinct existence-only F_OK path.
• Implement the supported permission checks with native APIs; return an explicit unsupported result for a mode whose meaning cannot be supplied reliably.
• Add fixtures distinguishing existence from denied/read-only write access, and verify invalid mode handling.
• Do not describe access checks as a race-free substitute for checking the subsequent open operation.

Related: #385. This concerns successful-but-false permission answers, separate from preserving native failure codes.

Audit status: identified by static source inspection; the scenarios above have not been executed during this audit. The permalink fixes the reviewed revision; this report does not claim the defect was introduced by #520.