Optimize generated Jackson deserialization

[schlosna]

Summary

• Add repository guidance for coding agents and Java stream conventions.
• Generate a type-first union deserialization fast path with mapper-local, lazy, thread-safe wrapper deserializer caching.
• Use prefix-only buffering for out-of-order union discriminators and a single discriminator switch.
• Populate generated builder maps directly during Jackson deserialization and declare Jackson Core dependencies explicitly.
• Regenerate the integration input fixtures.

Performance

Midpoints from two JVM forks with seven measured trials per fork:

Scenario	Baseline ns/op	Final ns/op	Baseline B/op	Final B/op
Union, type first	508	314	1,552	1,076
Union, out of order	492	489	1,552	1,576
Sealed union, type first	446	319	1,536	1,040
101-variant union, type first	480	328	1,552	1,056
101-variant union, out of order	510	517	1,552	1,640
32-entry map	3,019	2,474	8,224	6,552

Cold reader creation plus first read for the 101-variant union improved from about 490 microseconds and 134 KB to 300 microseconds and 22 KB. Serialization allocation remained unchanged; timing did not show a consistent shift.

The generated 101-variant deserializer decreased from 26,191 to 21,214 bytes compared with the initial reviewed implementation.

Testing

• ./gradlew :conjure-java-core:test
• ./gradlew build -x :conjure-java-server-verifier:test

The server verifier test is excluded because the local native verifier requires unavailable libssl.so.1.1.

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Description

Optimize generated Jackson deserialization

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💡 Improvements

• Optimize generated Jackson deserialization (#3029)