N2C: No way to fetch block contents by point (intersection)

[disassembler]

Problem

When a client has an intersection (a known block point: slot + hash), there is no way to fetch the block body for that specific block via the Node-to-Client (N2C) protocol. The same operation is possible via Node-to-Node (N2N).

The chain-sync mini protocol only allows advancing forward from an intersection — it gives you the next block after the intersection, not the block at the intersection. This creates a dead end:

• If the next block doesn't exist yet (i.e. the intersection is the chain tip), there is nothing to fetch.
• To fetch the block at the intersection, you'd need to roll back to its predecessor and request the next block — but the predecessor point is not always known to the client.

Workaround (client-side)

Store the previous block's slot and hash alongside each indexed block. To fetch a block by point:

1. Find the stored predecessor point.
2. Call FindIntersect on the predecessor.
3. Call RequestNext to receive the target block.
4. Verify the returned block matches the expected slot and hash.

This is fragile and requires extra index storage purely to compensate for a protocol limitation.

Proposed Solution

Expose a block-fetch-by-point capability as a mini protocol on N2C (analogous to how N2N allows fetching block ranges). A corresponding cardano-cli command would let users query a block by its point without needing to run a full chain-sync session.