feat(ax): apply AX compliance sweep — usage examples and predictable names
- Add // Example: usage comments to all Medium interface methods in io.go
- Add // Example: comments to local, s3, sqlite, store, datanode, node medium methods
- Rename short variable `n` → `nodeTree` throughout node/node_test.go
- Rename short variable `s` → `keyValueStore` in store/store_test.go
- Rename counter variable `n` → `count` in store/store_test.go
- Rename `m` → `medium` in store/medium_test.go helper
- Remove redundant prose comments replaced by usage examples
Co-Authored-By: Virgil <virgil@lethean.io>
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