POC: iceberg-testing conformance fixtures (submodule)

.gitmodules

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+[submodule "iceberg-testing"]
+	path = iceberg-testing
+	url = https://github.com/sungwy/iceberg-testing.git

conformance/README.md

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+# Conformance fixtures (POC — submodule wired, test deferred)
+
+This pins [`sungwy/iceberg-testing`](https://github.com/sungwy/iceberg-testing) as a
+git submodule at `iceberg-testing/`: language-neutral conformance fixtures for
+Apache Iceberg, modeled on `apache/parquet-testing`. Three surfaces:
+
+- `table-spec/types/` — type-string parse + exact re-serialize (canonical)
+- `table-spec/transforms/bucket/` — the Appendix B 32-bit bucket hash, as a
+  Known-Answer-Test, including byte-boundary decimals and a non-BMP string
+- `table-spec/delete-formats/` — positional and equality delete-file decode by
+  field-id
+
+A Java (JUnit) harness that walks these fixtures is **not included in this POC**. It
+was prepared in an environment without a JVM build, so rather than commit an
+unverified test, the wiring is left as a TODO. The PyIceberg fork has the worked
+example, including how a consumer keeps a local staged-adoption list for cases it
+does not yet satisfy: https://github.com/sungwy/iceberg-python/pull/1
+
+To consume: `git submodule update --init`, then walk `iceberg-testing/table-spec/**`
+and apply each surface README's assertion (parse + re-serialize; compute the bucket
+hash and compare; decode delete-file columns by field-id).
+
+The bucket Known-Answer-Test is the highest-signal surface to wire first: the
+expected hashes are taken from the spec's Appendix B, so it is a direct check of
+`BucketUtil` against the reference vectors plus byte-boundary decimal cases.