perf: zero-copy inbound TCP transport + test infrastructure consolidation

[st0o0]

Summary

• Bump servus.akka submodule to the unified rent-and-receive transport with zero-copy inbound TCP path, migrating all protocol layers from raw ReadOnlyMemory<byte> to WireBuffer
• Remove obsolete buffer size options (MaxRequestBufferSize, client-side ReceiveBufferSize) that are now transport-managed
• Consolidate ~10 duplicated test option factories into shared helpers (ServerOptionDefaults, TestClientOptions, DecoderEncoderDefaults, WireBufferTestExtensions), then migrate all H1.0/H1.1/H2/H3 server and client specs to use ServerTestContext and StreamTestBase

Changes

Transport (servus.akka submodule)

• Zero-copy inbound: transport rents buffers and hands ownership to the protocol layer via WireBuffer, eliminating defensive copies on the receive path

Protocol layer

• FrameDecoder, all state machines (H1.0/H1.1/H2/H3 client + server), body readers/pumps updated to consume WireBuffer instead of ReadOnlyMemory<byte>
• BufferedBodyReader and QueuedBodyReader adapted for new buffer lifecycle

Options cleanup

• Removed GaudiClientOptions.ReceiveBufferSize and GaudiServerLimits.MaxRequestBufferSize — buffer sizing now lives in the transport layer
• Simplified TransportBufferOptions and EndpointResolver

Test infrastructure (246 files, net -675 lines)

• New shared helpers: ServerOptionDefaults, TestClientOptions, DecoderEncoderDefaults, WireBufferTestExtensions
• Migrated all protocol specs to shared infrastructure, eliminating per-file boilerplate

Test plan

• Full unit + stage suite passes (dotnet run --project GaudiHTTP.Tests)
• Integration suites pass (Client, Server, End2End)
• Verify no public API surface regressions (acceptance tests)