feat: add SpringNullGuard — safe Spring nullability to JSpecify migration agent

[spargy4050v]

SpringNullGuard

SpringNullGuard is a migration agent for safely modernizing legacy Spring null-safety annotations to JSpecify.

Instead of performing a textual annotation replacement, it uses structural Java analysis and verification to preserve nullability semantics, including JSpecify TYPE_USE placement.

What it does

• Detects legacy Spring nullability annotations
• Performs structural Java migration using OpenRewrite
• Handles fields, parameters, returns, generics, arrays, and package defaults
• Fails closed on ambiguous or unsupported transformations
• Runs offline Maven baseline and proposal verification
• Requires independent verifier PASS before developer approval
• Performs post-apply structural and compilation verification
• Produces auditable migration evidence and traces

Real-world validation

A Bezkoder Spring Boot project is included under:

real-world/bezkoder-sb3/

The upstream project did not contain the target Spring nullability annotation, so one representative org.springframework.lang.Nullable annotation was intentionally seeded for evaluation and is explicitly disclosed in the submission.

Migration performed:

-import org.springframework.lang.Nullable;
+import org.jspecify.annotations.Nullable;

-@Nullable
-private String description;
+private @Nullable String description;

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