[0.11.0 ]feat: scaffold a typed PROPERTY_MAPPING example

[TKaltofen]

Summary

Add a minimal typed PROPERTY_MAPPING example to the generated plugin scaffold so third-party authors start with the supported property_spec() API and a tested configuration contract.

Motivation

The template currently gives plugin authors no PROPERTY_MAPPING example. Core authoring is becoming stricter and raw mapping dictionaries are no longer accepted, so authors need one small canonical pattern at the point where they create a plugin rather than having to infer it from core internals or copy stale downstream docs.

Tracked from mloda-ai/mloda#761 and its authoring-surface issue mloda-ai/mloda#776.

Code pointers

• placeholder/feature_group.py (or the current generated feature-group module)
• README.md
• docs/getting-started.md

Definition of done

• The scaffold contains a concise PROPERTY_MAPPING with property_spec(), one discriminator value space, and correct group/context classification
• The example uses only public imports from mloda.provider
• A test demonstrates valid and invalid configuration-based matching for the example
• Documentation explains when a plugin needs PROPERTY_MAPPING and links to the canonical core reference
• tox passes