Document DSAA3071 W2/W4/W6 source readiness

[yyuan308]

Summary / 概要

• Add a public-safe source-structure audit for DSAA3071 Weeks 2, 4, and 6.
• 新增 DSAA3071 W2/W4/W6 源材料结构审计，只记录匿名页结构、题目元数据和门禁状态。
• Record W4 (22 complete groups) and W6 (21 complete groups) as locally prepared but still blocked by privacy, blindness, and answer-preservation reviews.
• 将 W2 的非连续页组与 5 张附件候选页明确标为私有裁定阻塞，不自动猜测归属。

Data safety / 数据安全

• No student identities, answer text, scores, raw images, private layouts, or model outputs are tracked.
• 本 PR 不提交学生身份信息、作答、成绩、原始图像、私有 layout 或模型输出。
• Every week remains model_run_allowed: false; the report describes prerequisites, not an experiment result.
• 三周均保持 model_run_allowed: false；这是准备记录，不是模型结果。

Validation / 验证

• JSON parsing and page-count/group-count invariants passed.
• 已通过 JSON 解析与页数/页组数不变量检查。
• git diff --check passed.
• 公开文档为 ASCII 英文，避免中文编码问题。

Current limits / 当前限制

• W2 needs private page-ownership adjudication before a final layout can be generated.
• W4 and W6 need page-level grading-mark masks plus all three human reviews before gold, packets, or model runs.
• 在冻结逐题 gold 与模态比较前，还需决定满分 130 分或书面题子集的评估范围。