fix(ds4): retain wide heterogeneous feature capture

[davide221]

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

[cubic-dev-ai]

All reported issues were addressed across 35 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[davide221]

@cubic-dev-ai review the current head after the branch rebase. Please focus on correctness, maintainability, security, and compatibility with the updated base.

[cubic-dev-ai]

@cubic-dev-ai review the current head after the branch rebase. Please focus on correctness, maintainability, security, and compatibility with the updated base.

@davide221 I have started the AI code review. It will take a few minutes to complete.

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[davide221]

@cubic-dev-ai review the current rebased head. Please check correctness, maintainability, security, and interaction with the updated heterogeneous prefill path.

[cubic-dev-ai]

@cubic-dev-ai review the current rebased head. Please check correctness, maintainability, security, and interaction with the updated heterogeneous prefill path.

@davide221 I have started the AI code review. It will take a few minutes to complete.

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}