Add derived fields (FieldSpec.derivedFn) for Stores and Cube Views

[lbwexler]

Adds derived fields: a Field computed from the record's other values, read through a getter on record data so it is never loaded, parsed or written, and always current with its inputs. One declaration works on a plain Store, the Cube's own store, and a connected store, so apps can share a field dictionary across all three.

{name: 'notional', aggregator: 'SUM', dependsOn: ['qty', 'price'], derivedFn: d => d.qty * d.price}
{name: 'pnlBps', dependsOn: ['pnl', 'notional'], derivedFn: d => (d.pnl / d.notional) * 10000}

• FieldSpec.derivedFn + required dependsOn (may be []). A dependsOn naming a field the Store or Cube does not declare throws at construction. Store installs getters on its record-data prototype; raw parsing and modifyRecords ignore derived fields, getModifiedValues excludes them.
• On a CubeField the function runs wherever the field is not aggregated. With an aggregator it derives each leaf and rolls up (market value, notional). Without one it derives every level from that row's aggregates (PnL in bps, margin %, VWAP) - the ratio-of-sums case that previously required a custom aggregator and a full rebuild per tick. Parent rows get a getter rather than a slot. Derived fields may read other derived fields.
• Query pulls dependsOn fields in, as it does dimensions. On a data-only tick the View closes the producer's changedFields over dependsOn before diffing leaves (so a tick naming only price still re-sums a derived notional), and reports level-derived fields as changed to connected stores whenever an input moved.
• Use-case: incremental weighted averages and other ratios of sums (Support incremental aggregators that read other fields on the row (e.g. weighted average) #4747). {name: 'priceQty', aggregator: 'SUM', dependsOn: ['price', 'qty'], derivedFn: d => d.price * d.qty} plus {name: 'vwap', dependsOn: ['priceQty', 'qty'], derivedFn: d => d.qty ? d.priceQty / d.qty : null} keeps the View on the incremental dataOnly path with built-in SUMs - no custom aggregator, no dependsOnChildrenOnly: false, and no changes to the Aggregator / RowUpdate protocol. Leaf edits via Cube.modifyRecordsAsync() recompute the product automatically, since it is a getter rather than a stored value.
• 💥 Connected stores are now always projectionOnly - the View sets the flag, and projectionOnly: false or processRawData on a connected store throws. This is what lets a shared field definition carry a derivedFn harmlessly into a connected store: projections adopt values and never derive. Pulled forward from Data: calculated fields for Store and Cube View, connected stores via StoreConfig.view (#4620, #4637) #4638.

Perf: stores and Views without derived fields are unchanged from develop (benched, dense-record path). Those with them pay ~0.35µs per dense record/parent row created, for a __proto__-bearing clone that stays in V8 fast-properties mode. Verified with %HasFastProperties on real rows - the generated-class approach from #4638 produces dictionary-mode parent rows and was not used.

v88.0.0 shipped while this PR was open - its CHANGELOG entries now target 89.0.0-SNAPSHOT, with a new docs/upgrade-notes/v89-upgrade-notes.md for the connected-store change.

Not in scope: percent-of-total and other result-set-dependent values, which need lazy evaluation (see #4638). Follow-up noted: 'NULL'-aggregated fields still take a parent-row slot where a constant getter would do.

Toolbox PR: xh/toolbox#909

Hoist P/R Checklist

• Caught up with develop branch as of last change.
• Added CHANGELOG entry, or determined not required.
• Reviewed for breaking changes, added breaking-change label + CHANGELOG if so.
• Updated doc comments / prop-types, or determined not required.
• Reviewed and tested on Mobile, or determined not required. (Shared data layer - no platform code.)
• Created Toolbox branch / PR, or determined not required.

[lbwexler]

Follow-ups lifted from the (now closed) #4638 are tracked in #4748: read-only enforcement for derived fields, closing Store changedFields over dependsOn as the View now does, a canAggregateFn guard on level-derived fields, and doc carry-overs. None block this PR.