Fix network diff adjustment prediction algo

api/internal/bitcoind/rpc.go

@@ -205,6 +205,24 @@ func (c *RPC) GetBlockHash(ctx context.Context, height int64) (string, error) {
 	return out, nil
 }
 
+// BlockHeader is the subset of `getblockheader <hash>` we care about.
+type BlockHeader struct {
+	Hash   string `json:"hash"`
+	Height int64  `json:"height"`
+	Time   int64  `json:"time"`
+}
+
+// GetBlockHeader returns a block's header. Preferred over GetBlock when
+// only header fields are needed: getblock at verbosity 2 expands every
+// transaction in the block, which for a full block is megabytes of JSON.
+func (c *RPC) GetBlockHeader(ctx context.Context, hash string) (*BlockHeader, error) {
+	var out BlockHeader
+	if err := c.Call(ctx, "getblockheader", []any{hash, true}, &out); err != nil {
+		return nil, err
+	}
+	return &out, nil
+}
+
 // BlockVerbose2 is the subset of `getblock <hash> 2` we care about.
 // Verbosity 2 expands each tx into its full object so we can read the
 // coinbase output values without a second getrawtransaction call.