Fix mkdir symbolic mode parsing and -p -m leaf-only

[kelp]

Summary

• Add symbolic mode support (u=rwx, go=rx, a+rx, go-w, etc.) to mkdir's parseMode function. Previously only octal modes were accepted.
• Fix -p -m to apply the specified mode only to the leaf directory, matching GNU mkdir behavior. Intermediate directories now get default permissions (umask-based).
• Update unit tests and integration tests to match GNU semantics.

Test plan

• just it-util mkdir passes all 106 tests with 0 failures
• zig build test passes all unit tests
• Symbolic modes verified: u=rwx,go=rx (755), a+rx (555), go-w (noop from 0)
• Leaf-only mode verified: -p -m 700 sets 700 on leaf, default on intermediates