distribute/base.py: Artifact dataclass + Distributor protocol + idempotent
BEGIN/END snippet markers (upsert_block replaces a pattern's block in place so
re-distribution doesn't duplicate) + agnostic markdown body rendering from
SolutionPattern fields. BaseDistributor honours per-flavor body/target hints.
8 new tests; suite 110/110.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
