[REQ-125][BUG][cap:session-visual] 建立可比较的 Alpha 时间线生产性能基线

[jinjunnn]

Parent requirement

• Parent: [REQ-125] 会话页整页呈现 Alpha 自持体验并抗上游布局漂移 #538
• Found by verification: [REQ-125][VERIFY][cap:session-visual] 采集 seam 会话页与时间线组件对照已批稿的明暗视觉矩阵 #547

Problem

The C5 session timeline change merged without a pre-change production benchmark. The immediate pre-C5 Alpha workspace at 53531ff371abe49a2e29fc0bd0cd86f6e43a5f19 contains an empty .a-swk-timeline-host, so it cannot provide a meaningful “before” measurement for the new Alpha-owned timeline. The existing packages/app/e2e/performance/timeline harness measures the unchanged upstream app timeline and therefore is not valid evidence for the Alpha C5 delta.

This makes all three required benchmark rows terminal FAIL in #547; it must not be reported as PASS or silently omitted.

Acceptance criteria

• Add or designate a production-shaped, headless benchmark for the Alpha-owned session timeline using a fixed large-session fixture and fixed viewport.
• Record raw diagnostics plus three-run medians for cold open, 30-second streaming frame loss, and scroll/history-load latency and renderer memory.
• Pin the exact commit, fixture hash, browser/runtime versions, commands, and environment.
• Establish the resulting measurements as the future comparison baseline; do not retroactively claim a pre-C5 PASS.
• Keep the benchmark free of account credentials and real API keys.
• Update the canonical REQ-125 verification evidence and leave parent [REQ-125] 会话页整页呈现 Alpha 自持体验并抗上游布局漂移 #538 open until the benchmark contract is satisfied.

Verification contract

Follow packages/app/AGENTS.md and the performance harness guidance: production build, serial execution, raw diagnostics retained, and machine-dependent values reported rather than hidden behind an invented historical baseline.