Offline codegen: generate Java source at build time for JDK 21 runtime compatibility

[simbo1905]

Goal

Enable offline, build-time code generation so that JTD validators compiled via the codegen path do not require a JDK 24+ runtime.

Problem

Currently JtdCodegen.compile() uses the JDK 24+ ClassFile API (JEP 484) to generate bytecode at runtime. This means:

• The application must run on JDK 24+ to use the codegen path
• Generated bytecode is not persisted — it's regenerated on every startup
• No offline/aot compilation option exists

Proposed Solution

Add a Java source code emitter alongside the existing bytecode emitter. At build time (on JDK 24+), run a Maven plugin or CLI tool that:

1. Reads JTD schema JSON files
2. Generates plain .java source files containing validator classes
3. Those generated sources are compiled by the standard maven-compiler-plugin targeting Java 21
4. The resulting .class files are bundled in the application JAR
5. At runtime (JDK 21+), the pre-compiled validators are loaded via standard classloading — no ClassFile API needed

Architecture

Build time (JDK 24+)                    Runtime (JDK 21+)
┌─────────────────────┐                 ┌─────────────────────┐
│ JTD schema (.json)  │                 │ Pre-compiled        │
│       │             │                 │ validator.class     │
│       ▼             │                 │ (Java 21 bytecode)  │
│ JtdCodegen.emitJava()│                 │       │             │
│ → generates .java   │                 │       ▼             │
│       │             │                 │ JtdValidator.validate()│
│       ▼             │                 │                     │
│ javac -release 21   │────────────────▶│ ~9x faster than     │
│ → .class files      │                 │ interpreter         │
└─────────────────────┘                 └─────────────────────┘

Implementation Steps

1. Add EmitJava class (parallel to existing EmitNode/EmitType etc.) that generates Java source code instead of bytecode
2. Add JtdCodegen.emitJava(schema, packageName, className) method returning String source
3. Add jtd-codegen-maven-plugin (or reuse exec-maven-plugin) to run codegen during generate-sources
4. Generated sources go to target/generated-sources/jtd/
5. Standard compiler picks them up automatically
6. JtdValidator.compileGenerated() falls back to loading pre-compiled classes via Class.forName() instead of MethodHandles.defineClass()

Benefits

• JDK 21 runtime compatible — generated validators work on any JDK 21+
• No runtime codegen overhead — zero ClassFile API usage at runtime
• Offline/AOT friendly — works in restricted environments (containers, air-gapped)
• Debuggable — generated Java source can be inspected, debugged, decompiled
• Smaller attack surface — no runtime defineClass() calls

[simbo1905]

Completed via PR #148. The jdt2jar module now provides offline build-time compilation of JTD schemas into standalone validator JARs that run on JDK 21+. The generated bytecode is bundled in a JAR with a CLI entry point, and a distroless Docker image is available on GHCR for containerized usage.