Add automated vLLM and SGLang upstream synchronization

[Lanoxia]

Summary

This PR adds repository-owned synchronization for the OVG-maintained vLLM and
SGLang integration branches.

• Synchronize each integration branch with its official upstream main on a
  daily schedule or manual dispatch.
• Rebase the small OVG patch stack and update one stable automation branch.
• Run structural compatibility checks against the engine interfaces patched by
  kvcached before pushing any update.
• Open or update one compatibility PR per engine instead of creating duplicate
  PRs.
• Preserve JSON and Markdown reports for conflicts and failed checks so a
  maintainer or repair agent has actionable input.
• Skip an engine cleanly until its OVG repository is configured.

The PR also updates the SGLang allocator compatibility layer for the current
split allocator package and num_new_pages calling convention detected during
the upstream exercise.

Configuration

Set these repository variables after the OVG engine forks are ready:

OVG_VLLM_REPOSITORY
OVG_VLLM_BASE_BRANCH
OVG_SGLANG_REPOSITORY
OVG_SGLANG_BASE_BRANCH

The base branches default to kvcached-main. Cross-repository writes use the
OVG_SYNC_TOKEN secret; dry runs do not receive that token.

Validation

• 19 synchronization, compatibility, workflow-contract, and SGLang interface
  tests passed locally.
• A real dry run against Lanoxia/vllm:kvcached-main rebased cleanly onto the
  latest vllm-project/vllm:main; compatibility scan and three plugin tests
  passed.
• A real dry run against Lanoxia/sglang:kvcached-main rebased cleanly onto the
  latest sgl-project/sglang:main; compatibility scan and three plugin tests
  passed.
• Both exercises completed without pushing temporary automation branches.

Deployment boundary

The workflow is ready without private infrastructure. Final deployment only
requires the OVG engine fork locations and a token permitted to update those
repositories and open synchronization PRs.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.