Thermal Streams now evaluates counter-flow temperature fields reliably in either capacity orientation, including NTU=100. The previous forward formula could round a small terminal gap to zero and return a cold inlet 65 K too hot; the repaired normalized-integral evaluation agrees with an 80-digit reference within 5.7e-14 K across 15 saved cases.

• Evaluate a decaying exponential from the conditioned boundary instead of amplifying a tiny subtracted gap. Use the exact equal-capacity limit and cancellation-resistant effectiveness near C_r=1.
• Make LMTD symmetric and robust over wide positive input scales; normalize validated configuration inputs and reject nonrepresentable derived ratings.
• Add 29 meaningful conditioning/input regressions, including full-field high-precision comparisons and independent RK4 integration from the stable end. All 770 parametrized test cases pass locally.
• Regenerate the 48-frame white/no-grid GIF and clean figures. Save all 11,568 animated temperature samples, 250 heat-balance rows, the before/after comparison plot and its CSV/JSON data. Update equations, numerical limits, API and reproduction instructions.

These are numerical equation and implementation checks, not experimental validation. The GIF compares steady exchangers as area grows; terminal-LMTD checks retain their documented conditioning limits. Current main history and calendar disclosure are preserved.