feat: modernize integration workflows and add ClickStack observability

[lucarlig]

Summary

• reuse external conformance runtime state while preserving setup, execution, and cleanup failures
• expose semantic modern|legacy protocol modes and consistent builtin|external|fixture-direct lane selectors
• add external standalone load mode with a per-run mocked Redis snapshot that supports both dataplane routing schema generations
• replace separate observability services with ephemeral ClickStack/HyperDX: enabled by default for non-performance workflows and opt-in for load tests
• export control-plane traces plus dataplane traces and native HttpMetricsLayer metrics over OTLP
• shorten the README and document the command tree and representative workflows

Verification

• cargo fmt --all --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets (378 passed)
• built-in and external Compose configurations render successfully
• fixture-direct conformance, modern client against legacy servers: 2 passed, 38 expected failures, 0 unexpected failures
• built-in legacy load, 10 users at 2/s for 1m: 3,515 requests, 0 failures, 59.47 req/s
• external legacy standalone load with mocked Redis and ClickStack, 10 users at 2/s for 1m: 3,145 requests, 0 failures, 53.16 req/s
• ClickStack ingested http.server.request.duration, request/response body sizes, and active-request metrics from HttpMetricsLayer
• Rust CI and DCO pass on Linux, macOS, and Windows

Upstream checkout status

The built-in legacy live stack starts with ClickStack, but the current upstream live suite still expects hyphenated tool aliases while the gateway advertises underscore aliases, and that checkout lacks the test-protocol-compliance-gateway Make target. This PR does not patch control-plane implementation or tests.