Plugin framework: registry + PluginContext (plugins/__init__.py)

[GeoSpark]

The explicit in-tree plugin registry and shared context. See DESIGN.md §7.

Scope

• backend/plugins/__init__.py: @register decorator, discovery of in-tree plugins, PluginContext (raw dict + pystac.Item + loaded extensions.json metadata), enable/disable by name.
• Define the ValidatorPlugin Protocol: name: str, pure idempotent check(item, context) -> list[Finding].

Depends on

#2

Test approach

TDD. Register a fake plugin; assert discovery, name->source mapping, toggle on/off, and that running a plugin twice yields identical output (idempotence).

Acceptance

• Registry + context per DESIGN.md §7.
• Disabled plugins are skipped.