The lru_cache on get_block() caused three cache coherence bugs:
1. Removed blocks remained accessible — set_block_params would
modify a detached block object that no longer exists in the
flowgraph, so save_flowgraph would serialize stale state.
2. Re-created blocks with the same auto-generated name would
return the cached (removed) block instead of the new one.
3. Renamed blocks (via set_block_params id=...) left phantom
cache entries under the old name.
Fix: always look up blocks from the live flowgraph. Block lookup
is a linear scan of typically <20 blocks — no cache needed.
Raises KeyError instead of StopIteration for missing blocks.
