feat(fs): dirfd support

[Berrysoft]

Closes #659

cc: @rektide

[Copilot]

Pull request overview

Adds dirfd / *at-style filesystem operations to compio-fs via a new Dir handle type, enabling relative operations against an opened directory (useful for directory shuffling without recomputing absolute paths).

Changes:

• Introduces compio_fs::Dir (dirfd) with open_dir, open_file_with, rename, remove_*, and convenience read/write methods.
• Adds internal *_at plumbing (open_at, metadata_at, create_at) and Windows support via cap-primitives.
• Extends compio-driver with AsyncifyFd2 to run blocking work with two shared fds (used by some Windows dirfd ops).

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
compio-fs/tests/dir.rs	Adds basic tests for the new Dir API.
compio-fs/src/lib.rs	Exports dirfd module; adds shared-fd blocking helpers (spawn_blocking_with, spawn_blocking_with2).
compio-fs/src/dirfd/mod.rs	Public Dir API surface and convenience read/write helpers.
compio-fs/src/dirfd/unix.rs	Unix implementation using *at driver ops (openat/unlinkat/renameat/etc).
compio-fs/src/dirfd/windows.rs	Windows implementation using cap-primitives relative-to-handle operations.
compio-fs/src/open_options/mod.rs	Adds crate-private open_at used by dirfd.
compio-fs/src/open_options/unix.rs	Implements open_at by parameterizing OpenFile over an fd.
compio-fs/src/open_options/windows.rs	Refactors Windows open options to support conversion into cap-primitives options and adds open_at.
compio-fs/src/metadata/mod.rs	Adds crate-private metadata_at / symlink_metadata_at; updates Windows conversion path.
compio-fs/src/metadata/unix.rs	Adds metadata_at / symlink_metadata_at via PathStat with a provided fd.
compio-fs/src/metadata/windows.rs	Adds Windows *_at metadata via cap-primitives and refactors metadata representation.
compio-fs/src/utils/mod.rs	Adds DirBuilder::create_at / recursive create_dir_all_at.
compio-fs/src/utils/unix.rs	Adds Unix DirBuilder::create_at support.
compio-fs/src/utils/windows.rs	Adds Windows DirBuilder::create_at via cap-primitives.
compio-fs/src/file.rs	Uses the new blocking helper for some Windows file ops; adjusts permissions handling by platform.
compio-fs/Cargo.toml	Adds Windows dependency on cap-primitives.
compio-driver/src/op.rs	Adds AsyncifyFd2 opcode type.
compio-driver/src/sys/stub/op.rs	Registers AsyncifyFd2 with stub backend.
compio-driver/src/sys/poll/op.rs	Implements OpCode for AsyncifyFd2 in poll backend.
compio-driver/src/sys/iour/op.rs	Implements OpCode for AsyncifyFd2 in io_uring backend.
compio-driver/src/sys/iocp/op.rs	Implements OpCode for AsyncifyFd2 in IOCP backend.

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[Copilot]

Pull request overview

Copilot reviewed 21 out of 21 changed files in this pull request and generated 8 comments.

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[George-Miao]

LGTM