feat(p-pipe-cache): wire kei-cache into kei-pipe DAG executor
Optional per-step and DAG-level cache config in dag.toml:
[[steps]]
cache = { enabled = true, ttl_sec = 3600 }
OR
[pipe]
cache = { enabled = true, ttl_sec = 3600 }
Cache gated by AtomKind — only query/transform cacheable; command/stream
always re-invoke even with cache.enabled=true.
StepReport.source: Some('cache'|'fresh') | None shows cache outcome.
Constructor Pattern: extracted src/config.rs (CacheConfig + StepKind
+ TOML raw types + split_pipe_cache parser) + src/topo.rs (topo-sort)
to keep dag.rs under 200 LOC.
Tests: 8/8 (was 5, +3: cache-hit reuse, cache-disabled always invokes,
command-kind not cached even if enabled).
kei-cache 22/22 preserved.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
4a9dd98fde