#577 fix(ds4): retain wide heterogeneous feature capture

Summary

• keep batched DSpark feature capture enabled during sparse heterogeneous prefill
• preserve required capture boundaries for exact and unsupported hybrid paths
• cover the supported and fallback cases with focused unit tests

This PR is stacked on #575 and contains one focused change: 41 additions and 7 deletions across three files.

Validation

• test_deepseek4_unit: passed
• sparse heterogeneous context qualification: passed
• deterministic response hashes: matched
• local strict syntax and diff checks: passed