fix: createDirectorySync reporting success without creating directory

[tx3stn]

Description

Ran into an edge-casey issue where createDirectorySync can report success without creating the directory.

FileStat derives isDir / isFile / size from a struct stat without checking whether the stat() call actually succeeded. POSIX leaves the buffer unspecified on failure.
statFromPath zeroes it beforehand, so on platforms where a failed stat leaves the buffer untouched it reads back as "not a directory" and everything works.
Where a failed stat writes into the buffer, isDirectory() returns true for a path that doesn't exist, and the early return added in #107 then skips creation entirely.

Observed on linux/amd64 under Docker Desktop emulation on Apple Silicon. minimal C reproduction gets it wrong in both directions:

stat before: ret=-1 errno=2 isdir=1     # call failed, yet reports a directory
mkdir:       ret=0                      # created fine
stat after:  ret=0  isdir=0             # exists, yet reports not a directory

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Documentation improvement
• Performance optimization
• Test improvement
• Other (please describe)

Testing

• Tests pass locally (bazel test //...)
• Added/updated tests for changes (if applicable)
• Tested on multiple platforms (iOS/Android/Web/macOS as applicable)
• Manual testing performed (describe below)

Testing Details

Checklist

• Code follows project style guidelines
• Documentation updated (if needed)
• No breaking changes (or documented in description)
• Commit messages follow conventional format
• No secrets, API keys, or internal URLs included

Related Issues

Additional Context

[github-actions]

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