Add anonymous write scoped-session data scope

[eviltester]

Summary

• Adds opt-in anonymous write data-scope selection for scoped sessions.
• Supports fixed-name and resolver callback forms with creation policy.
• Keeps invalid scoped-session credentials rejecting instead of falling back to anonymous writes.
• Applies the resolved anonymous write scope before authorizers, validators, fixed routes, generated handlers, and callbacks.

Verification

• Full Maven verification passed during commit hook: mvn clean verify
• Local Maven install completed for API Challenges: mvn install -DskipTests

Closes #172

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