fix: stabilize table column widths during streaming

[callmeYe]

Description

Fix table column width jitter during streaming by applying a fixed table layout to Streamdown tables.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Related Issues

Fixes #
Closes #
Related to #

Changes Made

• Apply table-fixed to the standard table renderer so column widths remain stable while table content streams in
• Apply table-fixed to the fullscreen table renderer for consistent behavior there as well
• Add regression tests covering both the standard table path and the fullscreen table path
• Add a patch changeset for the fix

Testing

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

Test Coverage

Verified locally with:

• corepack pnpm --filter streamdown exec vitest run __tests__/components.test.tsx __tests__/table-fullscreen.test.tsx
• corepack pnpm --filter streamdown build

Screenshots/Demos

N/A

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

Additional Notes

This PR is intentionally scoped to the layout-stability fix only.

[vercel]

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