COG readiness: split SUPPORTED_FEATURES into writer.cog / reader.local_cog / reader.http_cog (#2286 PR 1/6)

[brendancol]

Part of #2286 (COG readiness/stability rollout).

Scope

Foundational, no behavior change. Split the COG entry in SUPPORTED_FEATURES so the writer, local reader, and HTTP reader can be promoted on independent tracks.

Required changes

• xrspatial/geotiff/_attrs.py:
  • Keep writer.cog at advanced.
  • Add reader.local_cog: advanced.
  • Add reader.http_cog: advanced.
• Add an inventory test that asserts all three keys exist in SUPPORTED_FEATURES (extend an existing inventory test or add a small new test file).
• Mention the split in the SUPPORTED_FEATURES docstring/comment block so the rationale is visible to future readers.

Out of scope

• Do not change any tier value other than adding the two new entries.
• Do not touch docs reference page or the COG notebook (PR 5/6 handle docs).
• Do not promote any feature to stable.

Acceptance

• SUPPORTED_FEATURES['writer.cog'], SUPPORTED_FEATURES['reader.local_cog'], SUPPORTED_FEATURES['reader.http_cog'] all resolve.
• Full geotiff test suite green.
• No production code path branches on the new keys yet.