feat(joint-react): make element/link data types unconstrained

[kumilingus]

Summary

Unconstrained data types

• Remove extends constraints from all framework generics so custom mapper users can use any data shape without extending FlatElementData/FlatLinkData
• Rename types: GraphElement → FlatElementData, GraphLink → FlatLinkData, GraphElementPort → FlatElementPort, GraphLinkEnd → FlatLinkEnd, GraphLinkLabel → FlatLinkLabel
• Generic parameters renamed from <Element extends GraphElement = GraphElement> to <ElementData = FlatElementData> (unconstrained with defaults)
• Default mappers retain extends constraints since they destructure known properties
• Remove unused GraphCell type and isGraphCell function

Introduce CellId type alias

• Introduce CellId = string type alias and replace all dia.Cell.ID and bare string (when used as cell identifiers) with CellId across the package (~35 files)
• Add as CellId casts at JointJS boundary points where dia.Cell.ID (string | number) narrows to CellId (string)
• Export CellId from the public API

Simplify hook signatures

• Replace unnecessary generic + conditional return types on useNodeLayout and useLinkLayout with clean function overloads
• Fix useLinkLayout context return type (was LinkLayout | undefined, now correctly LinkLayout)

Remove dead code (15 functions, 3 files deleted)

• is.ts: Setter, isSetter, isDefined, isAttribute, isDiaId, isLinkInstance, isCellInstance, assertGraph; make hasChildren non-exported
• object-utilities.ts: makeOptions
• link-utilities.ts: getLinkPortId, getLinkMagnet
• fast-equality.ts: shallowArrayEqual
• scheduler.ts: createScheduler
• Delete entire files: is-react-element.ts, noop-selector.ts, utils/clear-view.ts

Storybook

• Add with-render-link example with docs and toggle to switch renderLink on/off
• Simplify stress example: remove renderLink / custom mapper, use default JointJS link rendering

Test plan

• TypeScript typecheck passes
• All Jest tests pass (40 suites, 399 tests)
• ESLint passes (no new errors)
• Verify Storybook renders correctly
• Verify custom mapper example still works

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