refactor: extract packed capture_rows from the offline SGLang backend

[maocheng23]

Motivation

Stack 3/5 replacing draft #766. This PR is the capture-engine primitive the colocated runtime builds on: a packed variable-length capture API on the offline SGLang backend, plus DSpark capture-hook compatibility for dense targets.

Modifications

• Extract capture_rows(input_ids: list[list[int]]) -> (aux_rows, last_rows) from capture_eagle3 on OfflineSGLangCaptureBackend and expose it on OfflineSGLangCapture. One packed prefill captures variable-length rows without building padded tensors. capture_eagle3 delegates to it and keeps its exact request construction (set_extend_range, sglang 0.5.18), outputs, and pool clearing, so offline feature preparation stays byte-identical (unlike feat: add scalable colocated online training #766, no attention-mask trimming is bundled in).
• DSpark capture-layer setup falls back from the native set_dspark_layers_to_capture hook to the dense DFlash hook and logs the resolved hook. In stock SGLang only deepseek_v4 exposes the DSpark hook; dense targets such as Qwen3 serve the same auxiliary-hidden-state layout through set_dflash_layers_to_capture and previously failed outright. The hook table is one module constant (_CAPTURE_LAYER_HOOKS).

Related Issues

Splits #766. Stack: #781 teardown-abort ← #782 rank0-tracker ← #3 (this) ← #783 colocated-core ← #784 hybrid-shard.

Accuracy Test

• New tests/test_runtime/test_sglang_capture_hooks.py: hook preference, DFlash fallback, missing hooks, unknown method. Runs where sglang is installed (CI), like the existing sglang compat tests.
• test_sglang_0518_compat.py now inspects capture_rows for the request-construction contract.
• Behavior-preserving refactor for the existing offline path; exercised end to end by the colocated run reported in feat: colocated online training with in-process SGLang capture #783 (to be re-run on 0.5.18 after this rebase, see there).

Checklist

• Format your code according to the Code Formatting with Pre-Commit (black --check and isort --check-only).
• Add unit tests.
• Update documentation as needed (none needed — internal API).