Add Load and Parse methods to the metadata models

[glopesdev]

Device and IO pin metadata are now read through the model classes. DeviceMetadata gains Load, which takes a file path or a TextReader, and Parse, which takes the document as a string. A new PortPinMetadata type has the same Load pair for ios.yml. Parse reads a document that is already in memory, such as the device.yml embedded in a generated interface and exposed as Device.Metadata.

Both methods resolve the YAML merge keys that make schema reuse possible. The port pin path did not resolve them before, so an ios.yml that used a merge key would lose its inherited members, and the reader would report no error. No published ios.yml uses a merge key today.

These methods replace internal read helpers that were not reachable from outside the assembly, which made both harp-tech/toolkit and the documentation repeat the same six lines.

Breaking changes

FirmwareGenerator takes PortPinMetadata in place of Dictionary<string, PortPinInfo>. A caller that reads ios.yml through MetadataDeserializer and passes the dictionary must call PortPinMetadata.Load instead.

Two unrelated fixes

The generated GetDeviceMetadata now throws an exception that names the resource when a device package does not embed its device.yml. The failure would otherwise occur inside StreamReader, with nothing to identify the cause. This is the only change to generated output.

AssertExpectedOutput now fails when the reference output is missing. Seven comparisons on the errors.yml firmware row passed only because their reference files were absent.