BUG: Backport value-initialize singleton instances

[hjmjohnson]

Cherry-pick of 2f37d7a from main. Value-initializes singleton instances to avoid indeterminate values from default-initialization.

Backport for #6051 (Tier 1).

[greptile-apps]

Greptile Summary

This PR backports a single-line fix from main (cherry-pick of 2f37d7a3) that changes new T to new T{} in the Singleton<T> template function in itkSingleton.h. The fix ensures value-initialization instead of default-initialization, preventing indeterminate values for types without user-defined constructors (e.g., scalar types, POD structs, aggregates).

Confidence Score: 5/5

Safe to merge — minimal, correct bug fix with no side effects for types with user-defined constructors.

The single-line change from new T to new T{} is semantically identical for types with user-defined default constructors (the constructor is called in both cases) and is strictly safer for scalar/POD types (ensures zero-initialization vs. indeterminate values). No regressions are possible, and the fix correctly addresses the reported bug. The GlobalObject struct members already use brace-initialization, making this change consistent with the rest of the file.

No files require special attention.

Important Files Changed

Filename	Overview
Modules/Core/Common/include/itkSingleton.h	Single-character fix: new T → new T{} in Singleton<T>() ensures value-initialization for all instantiated types, eliminating potential indeterminate values for POD/scalar singletons.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Singleton<T>(globalName, deleteFunc)"] --> B["GetGlobalInstance<T>(globalName)"]
    B --> C{instance == nullptr?}
    C -- "No" --> F["return existing instance"]
    C -- "Yes (before fix)" --> D_old["new T\n(default-init: indeterminate for scalars/POD)"]
    C -- "Yes (after fix)" --> D_new["new T{}\n(value-init: zero-initialized for scalars/POD)"]
    D_old --> E["SetGlobalInstance + register deleteFunc"]
    D_new --> E
    E --> F

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[thewtex]

@hjmjohnson thank you!