fix: complete single-step progress bars

[galjos]

Closes #344.

Return 100% for a one-cycle progress bar without dividing by zero.

Tests:

• testProgressbar

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.82%. Comparing base (a15094d) to head (23dd5bc).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #349   +/-   ##
=======================================
  Coverage   89.82%   89.82%           
=======================================
  Files         293      293           
  Lines       11479    11479           
  Branches     3600     3600           
=======================================
  Hits        10311    10311           
  Misses       1126     1126           
  Partials       42       42           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[pq-perf-bot]

⚡ Performance (instruction count) — ✅ no regressions

per-benchmark breakdown

benchmark	base Ir	PR Ir	Δ
bondedForces	41.46M	41.46M	-0.00%	✅
boxTransforms	10.67M	10.67M	+0.00%	✅
constraints	11.38M	11.38M	-0.00%	✅
coulombKernel	5.80M	5.80M	+0.00%	✅
forceKernel	15.33M	15.33M	+0.00%	✅
integrator	32.17M	32.17M	-0.00%	✅
kinetics	9.53M	9.53M	+0.00%	✅
linearAlgebra	2.07M	2.07M	+0.00%	✅
nonCoulombPairs	5.48M	5.48M	-0.00%	✅
shiftVector	5.81M	5.81M	+0.00%	✅
virial	11.64M	11.63M	-0.07%	✅

_{Deterministic callgrind instruction counts vs the base branch; gated at ±2%. Not wall-clock.}