[codex] Implement matrix-avro v0.3.0

[perNyfelt]

Summary

Implements the matrix-avro v0.3.0 plan:

• Adds schema inspection APIs through MatrixAvroReader.schema(...) for file, path, URL, byte array, and stream sources.
• Keeps caller-owned streams open for InputStream and OutputStream APIs by adding close-shield wrappers.
• Fixes negative local-timestamp-millis reads and makes explicit timestamp-millis writes of LocalDateTime UTC-stable.
• Removes the writer schema cache to avoid stale schemas for mutated matrices.
• Adds ergonomic factories and shortcuts: AvroReadOptions.defaults()/named(...), AvroWriteOptions.defaults()/exactDecimals(), MatrixAvroWriter.writeExactDecimals(...), and AvroSchemaDecl aliases.
• Updates matrix-avro docs, release notes, v0.3.0 checklist, module version, and BOM entry.

Validation

• ./gradlew :matrix-avro:compileGroovy :matrix-avro:compileTestGroovy - BUILD SUCCESSFUL
• ./gradlew :matrix-avro:codenarcMain :matrix-avro:codenarcTest --rerun-tasks - BUILD SUCCESSFUL
• ./gradlew :matrix-avro:test - BUILD SUCCESSFUL, 96 tests passed
• ./gradlew test - BUILD SUCCESSFUL

Notes

matrix-avro/build.gradle still has codenarc { ignoreFailures = false }, so CodeNarc warnings fail the module build.