FSST: `fsst_compress` panics on cumulative output >2 GiB (i32 offset overflow)

[mprammer]

What happened

fsst_compress panics when cumulative output bytes cross i32::MAX:

Other error: Failed to convert sum of N and M to offset of type i32
   at vortex-array/src/arrays/varbin/builder.rs:62

The output buffer is built with VarBinBuilder::<i32> regardless of input size — encodings/fsst/src/compress.rs:72. A related per-string i32 ceiling sits on uncompressed_lengths at compress.rs:73,84.

Steps to reproduce

Regression test in #7832:

CI=1 cargo test --release -p vortex-fsst fsst_compress_offsets