Hrana WebSocket support (JSON, then Protobuf) #189

Goal

Let libsql-java speak Hrana 3 over WebSocket. A JDBC connection can then keep one multiplexed connection to sqld, instead of making one HTTP request per pipeline.

Phases

• Phase 1: JSON over WebSocket (hrana3 subprotocol). Issue: Hrana WebSocket: support JSON over WebSocket (phase 1) #187
• Phase 2: Protobuf over WebSocket (hrana3-protobuf subprotocol). Issue: Hrana WebSocket: support Protobuf over WebSocket (phase 2) #188

Cross-cutting requirements

Both phases share a seam that does not exist yet. Today LibsqlConnection depends directly on the concrete LibsqlHttpClient and on the HTTP baton as its source of truth for "is a stream open". WebSocket has no baton — the client assigns a stream_id. Phase 1 introduces a stream-level seam (open / execute / batch / close, with error mapping) implemented by both the HTTP client (baton) and the WebSocket client (stream_id), and migrates LibsqlConnection to it. Phase 2 reuses that seam and swaps only the frame encoding.

Sequencing

Phase 2 starts only after phase 1 is merged.

Source of truth and outside check

• Protocol: vendor/HRANA_3_SPEC.md, section "Hrana over WebSocket".
• Behavior: integration tests against the pinned sqld image (libsql.image in the root pom.xml), which is independent of this codebase.

Done when

• Both phases are merged.
• The integration tests run JDBC over WebSocket in both encodings against sqld.
• The HTTP transport is unchanged, and its conformance and integration tests still pass.