Publish a canonical xerial conformance statement #171

Problem

The repository has no single document that answers "have we reached conformance with xerial?" The answer is scattered across libsql-conformance/README.md, upstream-xerial_sqlite-jdbc.manifest, AGENTS.md, and the now-closed #54 queue. A contributor cannot tell at a glance what is covered, what is excluded, and why.

Proposed change

Write one canonical conformance statement (extend libsql-conformance/README.md or add a root CONFORMANCE.md) that states:

• The selected subset and the reference probe (-Pxerial-control), and its last green result (268 tests, 7 skipped at queue close).
• The three-layer contract: Hrana 3 is the source of truth for transport, values, metadata, batching, transactions, and error reporting; java.sql is the source of truth for the JDBC surface; xerial is a behavioral reference and conformance foundation, never a copy target.
• The compromise categories that make full parity impossible, each with a concrete example test: native extension API, sqlite-jdbc configuration surface, remote-model semantics (no local file, no shared connection, WAL locking), and build-flag dependence.
• The one addressable JDBC gap that remains: savepoints.

Acceptance criteria

• One document answers the conformance question end to end, linking to the manifest and the control profile.
• The statement names each compromise category and a concrete example test from each.