Fix: tighten loop bound in first_switch_pairs

[vdl299]

Summary

• Tighten loop bound in first_switch_pairs
• Remove redundant empty-check guard made unnecessary by the fix

Improvement

range(len(storage_stack)) was a misleading loop bound — the loop actually
ran at most len/2 iterations, relying on internal break guards to terminate
correctly. Replaced with (len(storage_stack)+1)//2 which reflects the exact
number of iterations needed. The first empty-check guard became redundant and
was removed.

Test plan

• python -m pytest tests/test_stack.py - 10 tests pass

[keon]

thanks for the contribution!