Function asEquation

src/compute-engine/boxed-expression/solve.ts:89–93

The equation `f = 0`, with any internal `_x` unknown displayed as `x`.

(f: Expression, x: string)

 * design, decision 7). Resolves a *decidable* guard against evaluation + the
 * assumption store:
 *   - guard evaluates to `True`  → the bare `root` (guard discharged);
 *   - guard evaluates to `False` → `null` (the caller drops the root — the
 *     solution-set pruning contract, decision 8);
 *   - otherwise (undecidable)    → `When(root, guard)`, retained until the
 *     guard becomes decidable.
 *
 * Numeric ratios therefore keep today's behavior exactly: the trig rules'
 * conditions already refuse to fire on a decidable-False ratio, so a numeric