Performance optimization: zero GenServer.call hot path, pluggable strategies, CI/CD

[nyo16]

Summary

• 4.3–5.8x faster get_channel — rewrote hot path to eliminate GenServer.call bottleneck. Channels stored directly in ETS with O(1) indexed access and
  lock-free atomics round-robin.
• Pluggable connection strategies — new Strategy behaviour with 3 built-in implementations: :round_robin (atomics), :random, :power_of_two
  (least-recently-used tiebreak). Custom strategies supported.
• 10 bottlenecks fixed — O(n) scaling eliminated, telemetry loop rewritten as proper GenServer, stale scaling lock detection,
  crash_after_reconnect_attempt replaced with max_reconnect_attempts, ETS concurrency flags added.
• New features — await_ready/2 for startup readiness, PoolState GenServer for ETS ownership, :persistent_term for zero-copy config reads.
• CI/CD pipeline — GitHub Actions with compile, format, credo, test, dialyzer, and auto-publish to Hex on v* tag push.

Benchmark Results

┌───────────────────────┬───────────┬───────────┬─────────────┐
│ Metric │ v0.2.x │ v0.3.0 │ Improvement │
├───────────────────────┼───────────┼───────────┼─────────────┤
│ get_channel (pool=5) │ 473K ips │ 2.03M ips │ 4.3x faster │
├───────────────────────┼───────────┼───────────┼─────────────┤
│ get_channel (pool=25) │ 343K ips │ 1.99M ips │ 5.8x faster │
├───────────────────────┼───────────┼───────────┼─────────────┤
│ 100 concurrent p99 │ 1,035 μs │ 439 μs │ 58% lower │
├───────────────────────┼───────────┼───────────┼─────────────┤
│ Memory/call (pool=25) │ 1.65 KB │ 0.72 KB │ 56% less │
├───────────────────────┼───────────┼───────────┼─────────────┤
│ Pool.status │ 1.47M ips │ 3.71M ips │ 2.5x faster │
└───────────────────────┴───────────┴───────────┴─────────────┘

Pool size scaling: pool=25 was 38% slower than pool=5 in v0.2.x — now only 2% slower (O(n) eliminated).

Test Plan

• 81 tests pass, 0 failures (mix test --exclude emulator)
• Strategy tests cover round_robin, random, power_of_two
• Existing pool scaling, telemetry, worker, config tests updated and passing
• Benchee benchmarks runnable via mix run bench/get_channel_bench.exs
• Run CI pipeline on PR
• Integration test with Pub/Sub emulator (optional, requires Docker)