Simple keto-enol tautomerism logic

**Background**
This ticket will describe the simplest instances of keto-enol tautomerism - where a single and double bond change positions resulting in a movement of a single hydrogen atom.

**Requirement**

- In a configuration H-X-Y=Z, where X and/or Z are highly electronegative atoms (O or N), a change in configuration of X=Y-Z-H is allowed if the valences of the elements allow that configuration of bonds.

**Examples**

| Number | Image |
| --- | --- |
| 03 | |
| 04 | |
| 05 | |
| 06 | |
| 12 | |
| 15 | |
| 16 | |
| 22 | |

The issue names no source files, tests, or entry points. Start by locating the existing tautomerism logic in the Indigo C++ codebase, then compare its behavior with the listed examples and valence rule. Done means the supported keto-enol transformations are implemented and covered by tests for the stated cases.

Written by the indexing model from the issue text.