Fixes the rc9 over-correction: forcing baseline for ALL nodes (including
caught-up ones) caused needless reseed cascades. Now only divergent nodes
(frontier > baseline) use baseline as the reseed seqno; at/below-baseline
nodes use frontier+1 so the leader picks cheap catch-up vs snapshot.
Also skips the HNSW graph checkpoint on SIGTERM when the shard is reseed-
pending: the in-memory index reflects suspect/divergent data the next boot
discards, so saving it risks a "Failed to read vectors" failure on the
post-reseed open. Durable checkpoints and WAL flush still run.
close_shared() gains a save_graphs bool; shutdown_inner_impl() is the
shared implementation; node.rs passes !reseed_pending.
44ec87871e