Add support for filling synoptic icons that point directly at an entity in ioc.yaml

It is correct that making `DCM` requires an entry in techui.yaml: you are making a new composite screen from multiple objects that match a prefix, along with a status PV.

However for a valve this is overkill, we don't want a composite screen or a status PV, we just want the icon to point to it.

I suggest there should be 2 sources of id for filling widgets:
- entries in techui.yaml like `DCM` register that id with the generated bob file path, and generated status pv
- entries in ioc.yaml files like `BL19I-VA-VALVE-01` lookup if they have an entry in techui mapping, then insert their prefix, generated bob file path, and listed status pv

## Acceptance Criteria
- You don't need to edit techui.yml when adding a new valve to a vacuum IOC, just add the icon to the synoptic and it will be filled by techui builder

Trace how techui.yaml entries currently fill synoptic widgets, including the generated bob path and status PV, then inspect how ioc.yaml entries are parsed. Add coverage for a valve entity referenced directly by a synoptic icon. Done means a new valve requires only its ioc.yaml entry and synoptic icon, without a techui.yaml edit.

Written by the indexing model from the issue text.