fix: WebUI Activity gate collides with host GPU work and stale incremental cache #2111

Problem

The WebUI installed artifact job (.github/workflows/ci.yml:632) on the GB10 runner lablup-dgxspark21 fails for two environmental reasons.

1. Lock collision. The Activity gate (:829-857) takes its own flock under $HOME/.cache/mlxcel/ci-locks, then fails closed at :833-837 if any GPU compute process exists. Development sessions on this host use gpu-lock (/usr/local/bin/gpu-lock, a flock on /tmp/gpu-lock-$(id -u)/lock), and the gate ignores that lock. Run 36982300183 attempt 1 (PR fix(tts): cast bf16 tables explicitly where the CUDA overlay demotes #2093) failed this way and passed on a rerun made while holding gpu-lock. The runner runs as uid 1000 with PrivateTmp=no, so it sees the same lock file.
2. Stale incremental state. The target dir $HOME/.cargo-target/mlxcel-webui-installed-ci (:659, profile test-fast with incremental = true, Cargo.toml:346-351, about 60 GB) hit a reproducible link failure: undefined serde_json generics in CGU c0jzxrmgkpaphvspsqkdqpw09. Run 36988516810 (PR fix(server): forward echoed reasoning under reasoning_content too #2094) failed at the build step on attempts 1-5 and passed on attempt 6 only after the incremental state was cleared.

Proposed fix

• When command -v gpu-lock succeeds, run the gate body under gpu-lock run --tag webui-activity --wait 600 -- ... inside the existing flock. On timeout, print gpu-lock status. With no gpu-lock on the host, keep the current behavior. The oom_score_adj=1000 that gpu-lock sets is acceptable.
• Set CARGO_INCREMENTAL=0 for this job only. Rejected: clean-and-retry on link failure, which hides the cause and costs a full rebuild. Other GB10 target dirs are out of scope.
• Document both behaviors in docs/webui-integration-matrix.md.

This edits the same block as open #1949 (quiet-host wait). Land it with #1949 or stack it on that PR, with the gpu-lock wait placed first. Coordinate with #1925.

Acceptance Criteria

• A run started while gpu-lock is held waits, which the job log shows, then passes after release.
• The job builds with CARGO_INCREMENTAL=0; the PR records the warm build time.
• actionlint passes and WebUI installed artifact is green on the PR.

Do it in one PR and keep pushes to a minimum, because each push costs a full GB10 gate cycle.