feat: add StructLikeSet for deduplicating StructLike rows

Adds StructLikeSet<bool kValidate = true>, a hash set for StructLike
rows backed by an internal arena allocator.

Key design points:
- Deep-copies inserted rows into a monotonic_buffer_resource arena;
  string data and nested struct/list/map scalars are fully materialized
  so the set owns its memory independently of the caller
- Transparent heterogeneous lookup: Contains() does not allocate a
  temporary key
- Hash and equality semantics match the Java reference implementation
  (String.hashCode, StructLikeHash, ListHash; float/double use
  canonical NaN bits and distinguish ±0.0)
- Schema validation (field count + scalar type) on Insert/Contains;
  can be disabled via kValidate=false (UncheckedStructLikeSet) when
  the caller guarantees conformance
- Internal Arena wrapper containers use std::pmr::vector

Author: wgtmac

src/iceberg/CMakeLists.txt

@@ -109,6 +109,7 @@ set(ICEBERG_SOURCES
     util/property_util.cc
     util/snapshot_util.cc
     util/string_util.cc
+    util/struct_like_set.cc
     util/temporal_util.cc
     util/timepoint.cc
     util/transform_util.cc

src/iceberg/meson.build

@@ -127,6 +127,7 @@ iceberg_sources = files(
     'util/property_util.cc',
     'util/snapshot_util.cc',
     'util/string_util.cc',
+    'util/struct_like_set.cc',
     'util/temporal_util.cc',
     'util/timepoint.cc',
     'util/transform_util.cc',

src/iceberg/test/CMakeLists.txt

@@ -116,6 +116,7 @@ add_iceberg_test(util_test
                  formatter_test.cc
                  location_util_test.cc
                  string_util_test.cc
+                 struct_like_set_test.cc
                  transform_util_test.cc
                  truncate_util_test.cc
                  url_encoder_test.cc

src/iceberg/test/meson.build

@@ -91,6 +91,7 @@ iceberg_tests = {
             'formatter_test.cc',
             'location_util_test.cc',
             'string_util_test.cc',
+            'struct_like_set_test.cc',
             'transform_util_test.cc',
             'truncate_util_test.cc',
             'url_encoder_test.cc',