`join` aggregate doesn't handle long content

[N1kto]

Bug Report

Steps to Reproduce:

1. Upload this simple (3 rows) CSV into perspective demo.
2. Group by group_by_col column
3. Select join aggregate function for data_col column

Expected Result:

1. No empty data_col cell for group2
2. Both group1 and group2 aggregated strings should contain all joined values

Actual Result:

1. group1 aggregate contains the first row value only, all following rows (only one in provided sample) were skipped
2. group2 has empty cell.

Environment (browser):

• @perspective-dev/client 4.0.0, 3.8.0
• Firefox Nightly 147.0a1 / Chrome 144.0.7525.0 (Official Build) canary (arm64)
• macOS 26.1

Additional Context:

Seems to be related to this limit:

perspective/rust/perspective-server/cpp/perspective/src/cpp/sparse_tree.cpp

Lines 38 to 39 in dc85be7

	// Tweet length
	const t_uindex MAX_JOIN_SIZE = 280;

Would be great if that limit was either relaxed or configurable. I assume that perspective was targeted to fin-tech or other "numeric-content" applications where rows being joined are unlikely to contain more than 280 chars. However pespective is super powerful and versatile, so it can be used in many other applications where cells may contain much longer content and this limit simply makes it unusable as missing data is unacceptable.