Derived fields follow-ups: read-only enforcement, Store changedFields closure, canAggregateFn guard, docs

[lbwexler]

Follow-ups to #4701 (derived fields), lifted from the review of #4638 before it was closed in favor of #4701.

1. Enforce read-only on derived fields

Store.modifyRecords() currently ignores writes to a derived field, and a grid column bound to one is still editable - an inline edit appears to succeed and then vanishes. Proposed:

• modifyRecords() throws when a modification names a derived field, e.g. Field 'notional' is derived and read-only - its value is computed at read time and cannot be modified.
• Column.isEditableForRecord() returns false for a column whose field is derived, regardless of the column's editable config.
• Doc FieldSpec.derivedFn accordingly (currently says writes are "ignored").

2. Close Store changedFields over dependsOn

#4701 (post catch-up) closes a producer's changedFields over dependsOn in View.dataOnlyUpdate, so a Cube tick naming only price still re-sums a derived notional. A plain Store has the same hole: updateData() with a StoreTransaction.changedFields hint naming only inputs is passed through to RecordSetDelta.changedFields, and GridTransactionManager.sortUnchanged() trusts it - a grid sorted on a derived column would not re-sort. Proposed: apply the same transitive closure in Store.updateData() before the hint reaches the RecordSet, and have the View reuse it (or a shared util) rather than keeping its own copy.

3. Guard canAggregateFn on level-derived CubeFields

CubeField throws on derivedFn + isDimension, but a level-derived field (no aggregator) that also sets canAggregateFn is accepted and the fn silently never runs. Throw at construction, as for isDimension.

4. Doc carry-overs

The FieldSpec.calculatedFn / Cube README prose from #4638 on read-by-name semantics (getters are not own properties, so never rely on own-property enumeration - use StoreRecord.getValues), keeping the fn pure and fast (it runs per cell paint and per sort comparison), and returning primitives or stable references so grid change detection can skip unchanged cells applies verbatim to derivedFn and should land in data/README.md / data/cube/README.md. The Cube README's WeightedAverageAggregator example should be replaced by the SUM/SUM/ratio decomposition (see #4747).