kvdb-rocksdb: optional per-column storage path

[x3c41a]

Adds ColumnConfig.path so a single column family's SST files can be placed on a separate directory, letting a large cold column live on a cheaper volume while the rest of the DB stays on the main disk.

When path is set, column_config gives that column a single cf_paths entry with a max target size, which pins the column to that directory (RocksDB does not spill elsewhere, and cf_paths overrides db_paths for that column). Empty by default, so existing databases and single-volume setups are unaffected. column_config now returns io::Result to surface an invalid path.

Motivation: Polkadot Bulletin Chain wants to keep its bulk indexed-transaction column on cheap SSD/HDD while consensus state stays on NVMe.

Note: cf_paths affects only where new SST files are written; relocating an already-populated column needs a compaction or resync.

Depends on rust-rocksdb exposing Options::set_cf_paths (rust-rocksdb/rust-rocksdb#1094); compiles and runs against that change locally.

Draft for discussion.

[x3c41a]

Heads up on CI: the compile failure (no method named set_cf_paths) is expected for now. Options::set_cf_paths was merged upstream in rust-rocksdb#1092 but is not in a published rocksdb release yet (latest crate is 0.24.0, which predates the merge). This PR will build once rust-rocksdb cuts a release including #1092 and the rocksdb dependency here is bumped to it. Keeping this as a draft until then. (Rustfmt is now fixed.)