add cache vs origin benchmark harness

[SISUinSea]

What changed

• Replaced the old simulated benchmark helper with a self-contained cache-vs-origin benchmark harness under tests/benchmark/*.
• Added an in-process dummy origin ASGI app that simulates a slow upstream fetch with fixed latency and deterministic payloads.
• Added a Mini Redis HTTP cache benchmark path that uses the real /v1/keys/* contract through app.main.create_app().
• Updated the benchmark CLI script to compare without_cache and with_redis_cache, including total time, average latency, origin fetch count, cache hits/misses, speedup, and improvement percentage.
• Rewrote benchmark tests to cover warmup behavior, cache miss-to-hit flow, payload consistency, state reset between runs, and summary formatting.

Why

• The previous benchmark only simulated a local cache function and did not exercise Mini Redis itself.
• For the demo, we need a short and repeatable benchmark that shows the relative difference between calling a slow origin directly and serving the same hot key through Mini Redis as a cache.
• This keeps protocol and store semantics unchanged while giving presenters a concrete benchmark flow that matches the branch scope.

Tests

• automated:
  • ./.venv/bin/python -m pytest tests/benchmark -q
  • ./.venv/bin/python -m ruff check .
  • ./.venv/bin/python -m ruff format --check .
  • ./.venv/bin/python -m mypy app
  • ./.venv/bin/python -m pytest
• manual smoke:
  • ./.venv/bin/python tests/benchmark/run_benchmark_demo.py --iterations 20 --warmup 1 --origin-latency-ms 20

Docs updated

• None in this slice.
• Shared docs, README, CI, and smoke artifacts were intentionally left unchanged because this step was limited to tests/benchmark/*.

Known limitations

• This benchmark is HTTP-only and does not cover RESP.
• The dummy origin runs in-process through ASGI transport, so this is a demo-oriented relative comparison rather than a full network benchmark.
• README, CI workflow, and presenter smoke flow updates are still pending for later work in chore/demo-bench-ci-readme.