Inline entity string verifier

[themuffinator]

Summary

• inline VerifyEntityString into g_local.hpp, keeping brace-depth tracking and safer EOF handling without a separate header
• rely on the shared inline verifier from g_spawn.cpp and update the override truncation test to include only existing sources
• escape the brace expectation message so format-string validation succeeds

Testing

• g++ -std=gnu++23 -I src -I src/fmt -I src/json tests/test_entity_override_truncation.cpp -o /tmp/test_entity_override && /tmp/test_entity_override (fails: std::sinf not available in this toolchain)

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Codex Task