fix: Special table owns readdir merge and write-redirect paths

[davydog187]

Summary

Leftover nits from #93 / #78 (JustBash.FS.Special). The special-file table is the right layer; these edges were still using a drive-by sort and a literal /dev/null match.

Changes

• Special.merge_children/2 only uniq-sorts when children(path) is non-empty. Ordinary FS.readdir/2 / ls order is the backend's again.
• Write redirects classify through Special.null?/1 after FS.resolve_path/2, so > /dev/./null is the same node as > /dev/null and skips Limit.check_file_size!. exec/2 still returns a shell result.
• Trailing-slash spellings (> /dev/null/) stay on the ordinary open path (bash: not a directory).
• No character-device type. test -f /dev/null remains true (VFS.Stat is still :regular).

Out of scope (investigated, not changed)

• ln -s /dev/null /link succeeds; cat /link is ENOENT. Special lookup is on the operand path, not after symlink follow, so the VFS backend sees no node.
• mv /dev/null /out is still cp-then-rm. rm is :eacces; the failed with drops the copied fs, so /out is not left behind. GNU rename fails without attempting the copy. Same user-visible result, different mechanism.
• test -f /dev/null is true. A char-device type is not needed for the two nits above.

Related Issues

Fixes #94

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Testing

• Added new tests
  • FS.readdir/2 of an unrelated dir preserves backend order (stream backend; VFS already sorts bounded lists)
  • merge_children/2 only sorts / and /dev
  • > /dev/./null with a payload over max_file_bytes exits 0 and does not raise
• All existing tests pass (mix test: 2 doctests, 62 properties, 5493 tests, 0 failures)
• mix compile --warnings-as-errors, mix format --check-formatted, mix credo, mix dialyzer, mix docs --warnings-as-errors

Checklist

• My code follows the style guidelines of this project
• I have run mix format
• I have run mix credo and addressed any issues
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• I have updated the documentation accordingly
• I have updated the CHANGELOG.md (for non-trivial changes)