[jiapingW]

Capture CI currently exercises a patched server, and plugin server metadata lacks the capture method and layer IDs needed to validate offline requests. This PR makes CI launch the native plugin in a separate cached sf-capture environment at SGLang commit c757c4bb7c55d1eb69fdb2bdc6e6fb8507acb7a0 and exposes that capture contract through /server_info.

It also keeps malformed NVLink free messages from discarding later valid messages, moves sink coverage to the plugin implementation, and updates the native scheduler fixture for the selected revision. Training dependencies remain on their existing versions until the final PR; the server dependency environment is isolated.

Validation: full pre-commit run --all-files passed. H20 plugin/sink regression: 60 passed, 1 skipped because the older training environment lacks native extension APIs. The native integration is exercised again in part 4 with the pinned SGLang source. Full GitHub GPU CI may still be queued.

Follow-up CI fix (c19f6b05): update the dependency-cache test fixture for both sf and sf-capture, exercise the current restore loop, verify separate contents and preserved symlinks, and require capture installation before saving the cache. All 8 CI dependency tests passed on Linux for each stack layer; the changed-file pre-commit checks also passed. Only the regression test changed.

Review and merge order:

1. [1/4] capture: prepare the native plugin runtime and CI server
2. [2/4] offline: prepare target features through the capture plugin
3. [3/4] capture: remove legacy SGLang patch support
4. [4/4] build: align training with the pinned SGLang revision

Each PR compares against the previous layer. Shared follow-up fixes are merged forward without rewriting published history. After merging a predecessor, retarget the next PR to main; if using squash merges, rebase the remaining stack before retargeting. The GPU workflow filters PRs targeting main, so later layers receive that CI run when retargeted. Lint runs on every layer.

This stack replaces #1062. Its combined feature changes match #1062 plus the lint fix; subsequent CI regression fixes are shared across the stack.

[curnane-lab]

LGTM. The separate capture env and native-plugin CI make sense.