fix: narrow sim2sim load dimension guard (#985)

[TATP-233]

Summary

• translate only explicit PyTorch size mismatch for ... state-dict failures
• preserve device, dtype, kernel, shape-operation, OOM, and unrelated value errors unchanged
• retain the original exception as the cause of translated diagnostics

Validation

• make test-all passed at a37ea086fa321c3a7ac208883c7cf9d08ca0b3bc
• 1660 passed, 27 skipped, 273 deselected, 1 xfailed
• benchmark smoke: 33/34 module-mode and 34/35 script-mode; MLX-only entries skipped

Closes #985