Rewrite Annotation Python thin wrapper

[dmartmillan]

Description
Reduce openvariant/annotation/annotation.py to a thin wrapper over _core.Annotation. Preserve all public properties for backward compatibility.

Files

• openvariant/annotation/annotation.py — REWRITE
  Preserved public API
  path, patterns, delimiter, format, columns, annotations (shim), excludes, structure (shim)

Definition of Done

• All existing property accesses still work
• annotations and structure shims return backward-compat dicts
• No import openvariant._core at module level (lazy inside __init__)

Documentation needed: No external API changes.

Tests needed:

• Existing tests/test_annotation.py must pass without modification