Add a non-blocking partial peek for layered streams

[samuel-williams-shopify]

Summary

• add IO::Stream::Readable#peek_partial for one non-blocking read attempt without consuming application data
• preserve any decoded byte in the stream read buffer
• mark the stream finished when TLS decodes close_notify
• return immediately available data, or nil when reading would block or reaches EOF
• cover clean TLS shutdown, abrupt termination, idle TLS, and buffered application data

HTTP/1 can use this primitive in its protocol-specific connection viability check before reusing an idle connection. This addresses the stale HTTPS connection behavior discussed in socketry/async-http#223 without changing the non-consuming readable? contract used by HTTP/2.

Validation

• 342 tests passed, 1823 assertions
• documentation coverage: 82/82
• RuboCop: 41 files, no offenses
• focused TLS tests: 4 passed, 9 assertions
• git diff --check passed