Feat/custom rendering composition

[fabric0de]

Summary

• Add typed framework-native header and cell renderer contracts for React, Vue, and Svelte.
• Add composition points for product toolbars and loading, error, and empty states.
• Keep product-specific UI outside the grid core while preserving framework-native rendering behavior.
• Add createSvelteDataGridRenderer for rendering Svelte components with typed context and stable props.
• Add React and Vue renderer tests, Svelte 4 compatibility coverage, external consumer validation, and task-oriented documentation.
• Rebaseline the required Svelte UI gzip ceiling to 21,000 bytes after measuring the renderer addition.

Svelte performance follow-up

• Render primitive Svelte header, cell, toolbar, and state values directly instead of creating a component boundary for every value.
• Use the RenderValue component only for explicit Svelte renderer descriptors.
• Reduce Svelte document nodes from 1,124 to 1,092 in the standard profile.
• Reduce Svelte document nodes from 1,437 to 1,395 in the wide profile.
• Rebaseline the measured Svelte-to-Vue document-node delta from 64 to 80.
• Retain the existing absolute DOM element, document-node, mounted-row, and mounted-cell budgets.

Public API

React

• renderHeader
• renderCell
• renderToolbar
• renderEmptyState
• renderLoadingState
• renderErrorState

Vue

• renderHeader
• renderCell
• renderToolbar
• renderEmptyState
• renderLoadingState
• renderErrorState

Svelte

• renderHeader
• renderCell
• renderToolbar
• renderEmptyState
• renderLoadingState
• renderErrorState
• createSvelteDataGridRenderer

Documentation

• Add a custom rendering guide for React, Vue, and Svelte.
• Add product composition guidance for toolbars and query states.
• Update framework package documentation and the public roadmap.
• Document the measured performance impact in CHANGELOG.md.

Verification

• Tests cover changed renderer and composition behavior.
• pnpm test && pnpm check passes.
• pnpm build && pnpm compatibility:check passes.
• Svelte 4 compatibility build passes.
• pnpm api:check && pnpm docs:build passes.
• pnpm release:consumer-smoke passes for all 10 packages.
• pnpm benchmark:structure:run passes all 40 structure checks.
• pnpm benchmark:core-filter:massive:run passes the 100K-to-1M row scaling budgets.
• pnpm benchmark:bundle:check passes all required package budgets.
• Svelte UI gzip is 20.06 KiB against a 20.51 KiB required limit.
• Public API and workflow documentation is updated.
• CHANGELOG.md is updated.
• No generated .benchmark-results/, .release/, credentials, or private data are included.