Hierarchical vertical merging should respect parent column grouping

What problem does this feature solve?

Problem Description

When using vertical cell merging, the current behavior merges cells with the same value even if their parent group in the previous column is different. This results in misleading groupings.

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Given this data:

Category	Priority	City
A	High	San Francisco
A	Medium	New York City
B	Medium	Paris
B	Low	Nice

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Current Behavior (Incorrect Merging)

Category	Priority	City
A	High	San Francisco
	Medium	New York City
B		Paris
	Low	Nice

Here, Priority: Medium was merged even though it's used under both Category A and Category B.
This is incorrect, because the values belong to different groups.

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Expected Behavior (Hierarchical Merging)

Category	Priority	City
A	High	San Francisco
	Medium	New York City
B	Medium	Paris
	Low	Nice

In this version:

• Priority is only merged within the same Category group.
• This respects the hierarchical structure of the data.

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Feature Request (☆▽☆)

Please support hierarchical vertical merging, where a cell in column n is only merged if all previous columns (0 to n-1) have the same value for the affected rows.

This will ensure merged cells accurately reflect group boundaries and avoid misleading groupings in complex data tables.

What does the proposed API look like?

We suggest adding an optional flag or configuration to the table that enables hierarchical merging, where cells are only merged if all columns before them have the same value.

Option 1: mergeStrategy: 'hierarchical'

const table = new VTable({
  ...,
  mergeStrategy: 'hierarchical',
});

The issue names the VTable constructor and the proposed mergeStrategy option but no files or tests. Start by locating the existing vertical-merging logic and option handling, then add coverage for the Category/Priority example; done means equal values merge only when all preceding columns match.