A constant is worth the current precision's digits, and a cached evaluation is recomputed when the precision context has changed

[Rafael-SOWNet]

Fixes #1367: pi and e were worth the digits of whichever precision context was set when Variable.ConstantList was built, and an entity's cached Evaled survived a change of MathS.Settings.DecimalPrecisionContext. So pi at 300 digits after an evaluation at 100 came back with 100, e^x at 300 came back correct to 100 (its base no longer equalled the cache's e, so Pow fell from "the exponential of the power" to a logarithm of the 100-digit base and an exponential), and a held expression evaluated at 100 digits and again at 500 answered from the first.

Two changes:

• ConstantList maps a name to Func<Complex> — () => MathS.DecimalConst.pi — so Constant.Value is the current context's digits every time. The registry's shape (a name is a constant exactly when it is a key) is unchanged.
• An evaluation epoch (Convenience.EvaluationEpoch, a static counter): the precision setting advances it on every Set and on every scope's Dispose (a Setting<T> opts in with AdvancesEvaluationEpoch, and only DecimalPrecisionContext does). Entity.Evaled records the epoch it was computed at beside its lazy slot and resets the slot when it has moved. The counter is one for every thread — settings are AsyncLocal, so a change on another flow costs a recomputation there, never a stale value.

Cost: one int per entity and one volatile read per Evaled. The gate's allocation rows move by +0.1% to +1.0% (8 bytes × the entities a row builds), all inside its band, so the baseline stands; time flat. The performance log has the entry (the 2075th).

Tests: ConstantsAndCachedEvaluationsFollowThePrecision (pi and a held sin 1 + pi at 100 → 300 → 100 digits, the 300-digit pi pinned against mpmath) and EToAPowerAtAHigherPrecisionAfterALowerOne (300 digits of e^0.37000000000001234 pinned against mpmath); both fail on master.

Four suites green (11,349 / 134 / 18 / 41).

Part of #1338.

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https://claude.ai/code/session_012sonx8iAspMiwRwokT1Ura