chore: prepare release 0.24.0

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This PR was created automatically. Merging it will create a new release for 0.24.0

Breaking Changes

Support concurrent chain.Manager reads via MVCC snapshots

The Manager previously serialized all of its methods -- including read-only ones -- with a single mutex, so heavy read load (e.g. from the syncer or subscribers) contended with block processing. The Manager now follows an MVCC scheme: writes accumulate in the Store's "scratchpad" -- which includes the tip state -- and are committed by Flush, at which point they become visible to "snapshots": self-contained, read-only captures of the committed state. Read-only Manager methods operate on snapshots, so they proceed concurrently with each other and with block processing, and never stall writers. bbolt supports snapshots natively via read-only transactions; MemDB now implements them with pin-aware copy-on-flush generations (CacheDB delegates to its underlying DB).

This is a breaking change for implementors and consumers of the chain.DB and chain.Store interfaces:

• Store is now Snapshot() (StoreSnapshot, func()) + Scratchpad() StoreScratchpad. StoreSnapshot carries the read methods plus TipState; StoreScratchpad embeds it and adds the write methods and Flush, with ApplyBlock/RevertBlock updating the scratchpad's tip. Implementations may flush autonomously between operations (e.g. to bound the size or age of the accumulated writes), handling errors internally; the Manager explicitly calls Flush to publish the new tip after processing blocks, before notifying OnReorg listeners.
• DB mirrors Store: Snapshot() (DBSnapshot, release func()) + Scratchpad() DBScratchpad. The scratchpad carries Bucket/CreateBucket/Flush/Cancel and must not flush autonomously; snapshots are read-only and remain valid until released, even across concurrent Flushes. A single DBBucket interface serves both sides; its Put/Delete methods may return errors on read-only (snapshot) buckets.
• NewManager no longer takes a consensus.State parameter, and NewDBStore/NewDBStoreAtCheckpoint no longer return one; the tip state is provided by the Store itself (via a snapshot's or scratchpad's TipState).
• Reads reflect committed state. Pruned blocks remain visible to read-only methods until the next commit, and blocks added to non-best chains remain invisible to them until the next commit. Blocks that extend the best chain are always committed before AddBlocks returns and before OnReorg listeners fire, so subscribers observe the tip they are notified of.

Fixes

• reject nonunique txid txes from mempool
• Use max of chain height and price table height when storing sectors.

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