[matt-edmondson]

Fixes #482

PH<T>.ToPOH() stored 14 − pH in a PH<T>, so every pH member misread it. The pOH of an acid reported IsBasic == true, printed as pH 11, and ToHydrogenConcentration() returned the hydroxide concentration.

This takes the issue's second option, which the triage comment preferred because it fits the generated logarithmic-scale pattern:

• A new POH<T> entry in logarithmic.json. It is generated like PH<T>, with FromHydroxideConcentration and ToHydroxideConcentration, and prints as pOH {0}.
• Chemistry/POH.cs adds ToPH(), which gives 14 − pOH at 25 °C.
• PH<T>.ToPOH() now returns POH<T>. POH<T> has no IsAcidic, IsBasic or pH formatting.
• The generated output and the four storage-type alias props are regenerated, so verify-generated sees no diff.
• docs/migration-guide-6.0.md covers the breaking return type, and CLAUDE.md adds POH to its list of logarithmic scales.

Tests

• POH_Of_An_Acid_Is_A_POH_And_Converts_Back: the pOH of PH.Create(3) is 11, prints as pOH …, gives [OH⁻] = 1e-11 mol/L, and converts back to an acidic pH 3.
• POH_From_Hydroxide_Concentration_And_Back: round-trips through the hydroxide concentration.
• With PH.cs reverted to main, the new test does not compile (CS0029: Cannot implicitly convert type 'PH<double>' to 'POH<double>'). With the fix it passes.
• Results on net10.0 locally:
  • Semantics.Test: 1946 passed, 0 failed, 14 skipped.
  • Semantics.Cpp.Test: 45 passed. The one remaining error is a missing older .NET runtime in this container.

Open PR #493 is also [major]. If both merge for the same release, its notes belong in the same migration-guide-6.0.md.

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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