use inverted index to serve labels/label_values instead of forward

[agavra]

Summary

The original approach was embarrassingly bad... I can't believe I merged it that way! I had to change it from the Aion implementation because we don't have explicit dictionaries.

• Optimizes labels() and label_values() endpoints to use inverted index keys instead of iterating through forward index
• Implements hybrid approach: forward index for filtered queries (match[]), inverted index for unfiltered queries
• Adds InvertedIndexKey::attribute_range() for efficient storage scans by label name
• Adds QueryReader::label_values() method that scans only keys matching the requested label

Test plan

• Existing tests pass (322 tests)
• Added unit test for attribute_range() key matching
• Existing integration tests for labels() and label_values() endpoints cover both filtered and unfiltered cases

Checklist

• Tests added/updated
• cargo fmt and cargo clippy pass
• Documentation updated (if applicable)

[agavra]

I'm going to optimistically merge this to unblock some perf testing, but it's definitely wroth specifying all the query paths in an RFC