Async Netty clients require an undocumented synchronous HTTP transport for polling

Summary

The README presents netty-nio-client as sufficient for async-only applications, but every async Alternator client also creates a separate synchronous HTTP client for /localnodes discovery and health probes.

A runtime containing DynamoDB plus Netty, but neither Apache nor CRT, cannot build the async client.

Current behavior

AlternatorDynamoDbAsyncClientBuilder.buildWithAlternatorAPI() independently detects:

1. Netty or CRT for the main async transport.
2. Apache or CRT for the synchronous polling transport.

SyncClientDetector.detect() throws when neither Apache nor synchronous CRT is available.

Relevant code:

• src/main/java/com/scylladb/alternator/AlternatorDynamoDbAsyncClient.java:791-808
• src/main/java/com/scylladb/alternator/internal/SyncClientDetector.java:23-70, 100-129
• README.md:75-96

The README currently says:

Async API only — choose Netty (recommended) or CRT

Netty cannot satisfy the synchronous polling requirement.

Impact

• Documented async-only dependency setup fails at client build time.
• Applications must discover an additional Apache/CRT dependency through runtime failure.
• Adding Apache solely for polling changes connection-pool behavior and footprint.

Expected behavior

Either:

• support discovery and health probes through the configured async transport, or
• clearly require and document a synchronous polling transport for async clients.

Acceptance criteria

• Netty-only async setup either builds successfully or fails validation with documentation that names the required extra dependency.
• README dependency examples match actual runtime requirements.
• Dependency/classpath tests cover Netty-only, Netty+Apache, Netty+CRT, and CRT-only combinations.
• Polling transport selection and preference order are documented.

Start with AlternatorDynamoDbAsyncClient.java:791-808 and SyncClientDetector.java:23-70, 100-129 to trace async transport detection and synchronous polling requirements. Compare those paths with README.md:75-96, then run the dependency/classpath tests for the Netty-only, Netty+Apache, Netty+CRT, and CRT-only combinations; done means runtime behavior, README examples, and polling preference order agree.