fix(joint-core, joint-react): override gridPatterns via getGridPatterns() instead of mutating static property

[Geliogabalus]

Summary

• Moves grid pattern color injection from a module-level mutation of `dia.Paper.gridPatterns` (static class property) into an overridable `getGridPatterns()` method on the Paper prototype
• Adds `getGridPatterns(): Record<string, Paper.GridOptions[]>` as a protected method on `dia.Paper` and updates the TypeScript declaration accordingly
• The `@joint/react` Paper preset overrides `getGridPatterns()` to substitute `--jj-paper-grid-color` into pattern colors at render time, rather than permanently mutating the shared static object

Motivation

The previous approach mutated `dia.Paper.gridPatterns` once at import time, making the CSS variable substitution permanent and side-effectful for any code that imported the preset. The new approach is non-destructive: each `GridLayerView` instantiation receives a fresh clone with the override applied, and the static property remains unmodified for downstream consumers.

Test plan

• Verify grid renders correctly with the default dot/line/fixedDot/mesh/jump patterns
• Confirm `--jj-paper-grid-color` CSS variable is respected by the grid layer
• Confirm `dia.Paper.gridPatterns` static object is no longer mutated after importing the `@joint/react` preset
• Check that subclassing `Paper` and overriding `getGridPatterns()` works as expected

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