Change structure of pvi PV

At the moment we have a structure like:

$ pvget -v BL45P-MO-PANDA-01:TTLOUT1:PVI
BL45P-MO-PANDA-01:TTLOUT1:PVI structure 
    structure record
        structure _options
            boolean atomic true
    structure pvi
        structure label
            string rw BL45P-MO-PANDA-01:TTLOUT1:LABEL
        structure val
            string rw BL45P-MO-PANDA-01:TTLOUT1:VAL
        structure val_delay
            string rw BL45P-MO-PANDA-01:TTLOUT1:VAL:DELAY

Which is very different from the underlying records:

$ pvget -v BL45P-MO-PANDA-01:TTLOUT1:VAL
BL45P-MO-PANDA-01:TTLOUT1:VAL epics:nt/NTScalar:1.0 
    string value ZERO
    alarm_t alarm 
        int severity 0
        int status 0
        string message 
    time_t timeStamp 2024-06-08 15:34:04.600  
        long secondsPastEpoch 1717857244
        int nanoseconds 600281953
        int userTag 0
    structure display
        string description TTL output value

It would be nicer to have value, alarm and timeStamp, and give it a structure ID. I suggest we do this:

$ pvget -v BL45P-MO-PANDA-01:TTLOUT1:PVI
BL45P-MO-PANDA-01:TTLOUT1:PVI epics:nt/NTPVI:1.0 
    structure value
        structure label
            string rw BL45P-MO-PANDA-01:TTLOUT1:LABEL
        structure val
            string rw BL45P-MO-PANDA-01:TTLOUT1:VAL
        structure val_delay
            string rw BL45P-MO-PANDA-01:TTLOUT1:VAL:DELAY
    alarm_t alarm 
        int severity 0
        int status 0
        string message 
    time_t timeStamp 2024-06-08 15:34:04.600  
        long secondsPastEpoch 1717857244
        int nanoseconds 600281953
        int userTag 0
    structure display
        string description PVs making up Interface for TTLOUT1

This would require:

Some structure to get the metadata into the root. Something like this when the first call to add_pvi_info_to_record happens:

    pvi_pv = f"{block}:PVI"
    description = builder.StringOut(
        pvi_pv + ":DESCRIPTION",
        initial_value=f"PVs making up Interface for {block}",
    )
    description.add_info(
        "Q:group",
        {
            pvi_pv: {
                "+id": "epics:nt/NTPVI:1.0",
                "display.description": {"+type": "plain", "+channel": "VAL"},
                "": {"+type": "meta", "+channel": "VAL"}
            }
        },
    )

https://github.com/PandABlocks/PandABlocks-ioc/blob/8e25d64cc97cfbf242a84c6ccbe600a39ed41c41/src/pandablocks_ioc/_pvi.py#L63 to start with value. not pvi.

PandABlocks / PandABlocks-ioc

Change structure of pvi PV #120