add main-chain block indicator to /txmeta response

[oskarszoon]

Problem

GET /api/v1/txmeta/{txid}/json returns parallel arrays of every block a tx appears in (blockIDs, blockHashes, blockHeights, subtreeIdxs, subtreeHashes) with no way to know which entry is on the current main chain. Callers must perform a second round-trip per blockHash to determine the canonical block.

Asked for in discussion #960. Caller use-case: exchange / wallet wants the canonical confirmation height for a tx without N extra calls.

Proposal

Add a single field to the response identifying the main-chain entry:

{
  "blockIDs": [1849362, 1849364],
  "blockHashes": ["...orphan...", "...main..."],
  "blockHeights": [1737884, 1737885],
  "subtreeIdxs": [0, 0],
  "subtreeHashes": ["...", "..."],
  "mainChainIndex": 1,
  ...
}

mainChainIndex = index into the parallel arrays for the main-chain block, or -1 if the tx is only in orphans / mempool.

Compute via BlockchainClient.CheckBlockIsInCurrentChain per block ID (small N — usually 1).

Alternatives considered

• Filter response to only main-chain block: breaks diagnostic value (callers debugging forks need to see all blocks).
• Boolean per entry (onMainChain: [false, true]): more verbose, no advantage over an index.

Tests

• TX in 1 main-chain block → mainChainIndex: 0.
• TX in fork + main chain → index points to main block.
• TX in orphan only → mainChainIndex: -1.
• TX in mempool → mainChainIndex: -1, empty block arrays (existing behaviour).