Extension heuristics plugin + bundled extensions.json (plugins/extensions.py)

[GeoSpark]

Port the extension heuristics from eodh-validator as the first registered plugin, and ship the metadata it needs. See DESIGN.md §6, §7.

Scope

• backend/plugins/extensions.py: port test_item_extensions logic — maturity warnings, Item-vs-Collection scope, declared-but-unused extensions, used-but-undeclared prefixes, unknown prefixes, nested-property advice.
• Emit Finding(category=extension, confidence=heuristic) (normative only where genuinely spec-grounded).
• Ship pre-built data/extensions.json as package data. Generation (get_extensions.py) stays offline — do NOT fetch at validation time.

Depends on

#7

Test approach

TDD. Fixtures per heuristic branch; assert tailored messages and heuristic confidence.

Acceptance

• All eodh-validator extension branches reproduced.
• extensions.json bundled and loaded via PluginContext.