[farhan-syah]

Closes #338.
Closes #395.
Closes #396.
Closes #397.

Error classes (#338)

• A response cell of the wrong shape raises 22000 (data exception), not XX000.
• A RETURNING row that cannot be decoded or shaped fails the statement with its own SQLSTATE. No fallback row is sent.
• Calvin RETURNING shapes its rows once per batch, with the session's sequences. nextval() in a RETURNING list evaluates.
• Native SQL Calvin RETURNING honours the RETURNING list.
• HTTP NDJSON error lines carry the error code.

Vector codecs (#395, #396, #397)

• Each sealed segment carries one codec: none, SQ8, PQ, or a graph codec (RaBitQ, BBQ, binary, ternary, OPQ).
• The codec builds on the builder thread with the graph.
• A codec build error installs the graph without a codec and reports the error. A memory-budget error retries with backoff, from 1 s to 60 s, 8 times at most.
• DDL refuses a PQ, OPQ or graph-codec shape the codec cannot build, with 22023.
• Ternary and OPQ indexes are searchable.
• One rerank rule on every path: the pool is top_k × oversample, the beam is max(ef_search, pool), then exact rerank.

Behaviour changes

• oversample defaults to 3.
• target_recall widens the beam. It must be in (0, 1].
• A search-time quantization hint that differs from the index codec raises 22000.
• The default VectorQuantization is Sq8.

Breaking changes for embedders (nodedb-lite)

• CodecName and VectorQuantization gain variants. VectorQuantization is no longer #[non_exhaustive], so exhaustive matches need the new arms.
• BuildRequest, BuiltSegment, complete_build, complete_rebuild and seal change signatures. complete_build and complete_rebuild return InstallOutcome.
• OpqCodec::train returns Result.
• IVF search takes an oversample argument.

Licence headers

• 28 files in Apache-2.0 crates carried a BUSL-1.1 SPDX header. They now say Apache-2.0.

Tests

• Full suite: stage 1 24268/24268, stage 2 535/535.
• New tests cover cell-shape SQLSTATEs, Calvin RETURNING errors and nextval(), codec build errors and retry, compaction id remapping, and the code-stage pool.