Port branchfs to macOS

[nitin966]

Description

Port branchfs to macOS with verified integration tests and stable control interface

Testing

All existing tests pass

[congwang-mk]

Could we separate the platform specific code? For example, src/platform/linux.rs and src/platform/macos.rs behind a src/platform/mod.rs that re-exports the right impl via #[cfg]. Move into it:

• Ioctl constants
• Mount option construction
• init() capability setup
• Passthrough open logic
• access() impl
• Rename flag handling

[congwang-mk]

Thanks for your update. Some additional reviews

[nitin966]

Changes in recent commit

• Made linux 256, mac 0 and controlled via the platform specific config.
• Reverted to use inherit()
• Removed logging statement

All tests pass.

running 22 tests
test test_branch_isolation ... ok
test test_commit_base_file_modification_visible_at_mount_root ... ok
test test_commit_changes_visible_at_mount_root ... ok
test test_commit_deletion_visible_at_mount_root ... ok
test test_create_read_write_delete ... ok
test test_mkdir_and_nested_files ... ok
test test_nested_branch_inheritance ... ok
test test_readdir_branch_inherits_main_delta_files ... ok
test test_readdir_nested_branch_inherits_parent_delta ... ok
test test_readdir_respects_tombstones ... ok
test test_rename_file_cross_dir ... ok
test test_rename_file_same_dir ... ok
test test_rename_in_branch_then_abort ... ok
test test_rename_in_branch_then_commit ... ok
test test_rename_nonexistent_fails ... ok
test test_rename_overwrite_existing ... ok
test test_symlink_abort ... ok
test test_symlink_base_visible ... ok
test test_symlink_commit ... ok
test test_symlink_create_in_branch ... ok
test test_symlink_delete_in_branch ... ok
test test_symlink_isolation_between_branches ... ok

test result: ok. 22 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 28.46s