[Feature Request] Add Save-Area Size Helper API (Caller Preallocation)

[k8ika0s]

Proposed PR branch: qe/issue-09-save-area-size-api

Category: Feature Request (performance + usability)

Problem

• Save-area allocation requirements are non-obvious to integrators.
• Precomputing sizes enables reuse and avoids repeated allocation overhead.

Justification / why this is needed

• Like work-area sizing, save-area sizing is needed to avoid hidden runtime
  allocations, especially when a caller wants strict memory control.
• Helps keep higher-level integration code smaller and less error-prone.

Proposed change

• Add zdnn_get_save_area_size(...).
• Add tests.
• (Optional follow-up) Add README guidance where save-area usage is currently unclear.

Acceptance criteria

• Helper returns required bytes (return 0 on invalid inputs).
• Tests validate sizing and invalid argument handling.

Test plan

• make test (expects tests/testDriver_save_area_size_apis.c to pass).

References

• Key files: zdnn/save_area.c, zdnn/zdnn.h, zdnn/zdnn.map,
  tests/testDriver_save_area_size_apis.c.