[PureGo] Add Avro record format (avro build tag)

[irinatomic-db]

PR stack

• avro/proto-surface [MERGED]
  • avro/purego <- this PR
  • avro/rust-core [MERGED]
    • avro/ffi-c-abi
      • avro/go
      • avro/cpp
      • avro/dotnet
    • avro/java
    • avro/python
    • avro/typescript

What changes are proposed in this pull request?

Adds the Avro record format to the pure-Go SDK, behind the avro build tag. Ephemeral streams only; feature in development. Default builds are unchanged — tagged files aren't compiled and no Avro dependency is pulled in.

Declare the writer schema once at stream creation with WithAvro(schemaJSON); it is parsed and validated there, so a malformed schema fails fast. Records are then ingested in one of two ways: pre-encoded bytes via IngestRecordOffset([]byte) / IngestRecordsOffset([][]byte) (raw Avro datums the caller already encoded), or record objects the SDK encodes via IngestAvroRecordOffset(AvroRecord) / IngestAvroRecordsOffset([]AvroRecord), where AvroRecord is a map[string]any.

AvroRecord covers every Avro type in the Avro-to-Delta mapping, including unions (via the Union("branch", v) helper), fixed, decimal, and logical types, using hamba's native value conventions documented on the AvroRecord type.

Implementation notes: object encoding uses github.com/hamba/avro/v2, imported only in //go:build avro files, so the pre-encoded bytes path needs no Avro library and stays available without the tag. The writer schema is parsed once at stream creation and reused across recovery, with no per-record parsing. Object-encode failures return non-retryable errors.

How is this tested?

cd purego

# Default (feature off) — tagged files skipped, no Avro dep
gofmt -l . && go build ./... && go vet ./... && go test ./...

# With the avro tag (all-types encode test, fail-fast test, gated WithAvro tests, e2e)
go build -tags avro ./... && go vet -tags avro ./... && go test -tags avro ./...

# Examples are a separate module; build the avro example with the tag
cd examples && go build -tags avro ./...

CI: ci-purego.yml has an avro job covering all of the above.