[Epic] faster-redis: redis-py compatibility parity for the Zig-native client

[justrach]

Bring the Zig-native faster-redis client to honest, test-backed compatibility with redis-py.

Goals:

• Replace broad parity claims with measurable compatibility targets.
• Close gaps in command normalization, kwargs handling, response semantics, pipeline behavior, and connection/session behavior.
• Build a live-Redis compatibility harness so future parity claims are backed by reproducible tests.

Non-goals:

• Benchmark-only wins that rely on reduced semantics.
• Marketing claims that exceed the tested behavior.

Initial scope:

• Command-name and argument normalization for fallback and pipeline paths.
• Keyword-argument support for high-value command families, or explicit unsupported-path errors.
• Response callback parity for common commands where redis-py normalizes wire responses.
• Pipeline and transaction semantics parity.
• Compatibility test matrix against redis-py on live Redis.
• Documentation updates that clearly distinguish raw-command fallback from true compatibility.

[justrach]

Blocks: #70, #71, #72, #73, #74