feat: update dgrid integration and add new index-preview for dgrid2

[GordonSmith]

Checklist:

• The commit message is properly formatted and free of typos.
  • The commit message title makes sense in a changelog, by itself.
  • The commit message includes a "fixes" reference if appropriate.
  • The commit is signed.
• The change has been fully tested:
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• There are no similar instances of the same problem that should be addressed
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Testing:

[Copilot]

Pull request overview

Updates composite and demo integrations to use @hpcc-js/dgrid2 and introduces a new preview HTML entrypoint for dgrid2.

Changes:

• Switch composite’s table integration from @hpcc-js/dgrid to @hpcc-js/dgrid2 (including chart type mapping and dependency update)
• Add packages/dgrid2/index-preview.html to support an importmap-based preview flow
• Adjust demo/index pages and Vite config to align with the updated integration

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
packages/dgrid2/vite.config.ts	Adds a define override for process.env.NODE_ENV
packages/dgrid2/index.html	Updates the module import path used by the quick test page
packages/dgrid2/index-preview.html	Adds a new importmap-based preview page for dgrid2
packages/composite/tests/composite.browser.spec.ts	Removes dgrid shim loading/assertion from browser tests
packages/composite/src/MultiChart.ts	Switches dynamic imports/mappings from dgrid to dgrid2
packages/composite/package.json	Replaces @hpcc-js/dgrid dependency with @hpcc-js/dgrid2
packages/composite/index.html	Removes shim script and adjusts demo timing to exercise TABLE
packages/composite/index-preview.html	Removes shim script and adjusts demo timing to exercise TABLE

[GordonSmith]

Closing in favor of #4531