Function simplifyLog

src/compute-engine/symbolic/simplify-log.ts:31–38

(x: Expression)

 * `because` label in the cost gate; it now travels with the step as a
 * `purpose: 'transform'` tag.
 *
 * The exempted set is the `ln(...)` / `log_...` label prefixes the cost gate
 * originally matched, plus the two `e^(… + y)` sums named in
 * `EXP_OF_LOG_SUM_LABELS`. Those two had been left out only because the
 * original whitelist did not name them: they were passing on the gate's growth
 * tolerance instead (`e^(x + ln x)` scores 14, `x·e^x` scores 18). Splitting a
 * logarithm out from under an exponential is a preferred rewrite whatever it
 * scores, so it is now tagged outright.
 *
 * They are matched by exact label, NOT by an `e^(ln(` prefix: sibling rules in
 * this file label themselves `e^(ln(x) * y) -> x^y` and