refactor: narrow BasicFile interface — apply ISP to Touch and Directory

Touch and Directory inherited content/from from BaseFile but had no use
for them. Directory worked around this by repurposing content() as a
source-directory path store, which was semantically misleading.

Changes:
- Move content/from down into BaseFile (only File and Symlink need them)
- Move evaluate_absent! up into BasicFile (Touch and Directory need it)
- Move UnsupportedOperation up into BasicFile (validate raises it there)
- Re-parent Touch and Directory to BasicFile directly
- Add Directory#source accessor to replace the content() misuse
- Update DSL#directory to call d.source(...) instead of d.content(...)

All 29 tests continue to pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions