feat(common): Sd0::Compress, a default Sd0 and room for chunks that don't shrink

Sd0 gets a default constructor and Sd0::Compress(data), the raw sd0 bytes of
some data (what the client reads for UGC files it downloads without 3D
services). FromData compresses into a heap buffer big enough for a chunk that
grows when deflated (it used a 256 KiB stack buffer, so random data failed and
workers carried a large stack frame), and drops a half-written result.

Tests: chunks as the 1.10.64 client inflates them, incompressible data, empty.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/dCommonTests/CMakeLists.txt

@@ -29,6 +29,7 @@ set(DCOMMONTEST_SOURCES
 	"PropertyReputationRulesTests.cpp"
 	"BindAddressTests.cpp"
 	"TrafficStatsTests.cpp"
+	"Sd0Tests.cpp"
 )
 
 add_subdirectory(dEnumsTests)