feat(digest): 探索图冷节点压缩(cold-digest)

把老且长期不活跃的意图/事实折叠成 digest 节点进 graph_overview 展示，
原始节点永久保留、按 id 可完整还原（存储无损、只压呈现、折叠可逆）。

- 数据模型：KindDigest 节点 + RelCovers 边（真相源）；新列 content_version、
  cold_since_round（节点级）、round_no（任务级 planner 轮次）；均带旧库迁移。
- 纯算法(coldgraph.go)：冷热判定(逆向可达 + 任一活分支即热) + R=6 轮防抖 +
  连通分量分组(§3) + §3.1 同父兄弟归组(救"热 hub+扁平死叶") + §5.3 签名。
- 后台压缩(compaction.go)：minor 折未覆盖冷块(tiered)、major 回源重压合并碎片；
  签名幂等复用 body；提交时活跃度复核(绝不覆盖已复活节点)；60s 冷却 + 互斥。
- 概览折叠(§6)：recent_* 剔除已折成员 + 渲染时复活检查；cold_digests + 按资产
  索引 cold_index(§6.2)；expand_digest/expand_index 还原；covered_members 反查
  悬空血缘 id；60 兜底截断均衡保 live。
- planner 每轮 OnPlannerRound 维护戳记并触发后台压缩，绝不上热路径。

纯算法单测覆盖 §附/§3.1/§2.3/§5.3；已对 437 节点真实图验证：热 86、冷候选 324、
§3.1 分组 47 块 + 14 孤立冷点（对齐设计文档实测校验）。

db/constants.go

@@ -8,6 +8,15 @@ const (
 	KindFact    = "fact"    // a worker's exploration result/conclusion (incl. negative results), tied to its intent
 	KindFinding = "finding" // a confirmed vulnerability (report_finding), distinct from a fact
 	KindHint    = "hint"
+	KindDigest  = "digest" // a compressed fold of cold intents/facts (cold-digest-spec §1); lossless — members kept, restorable by id
+)
+
+// Digest node states (kind='digest'). A digest is 'active' while it renders in
+// graph_overview; major compaction retires a merged-away segment to 'superseded'
+// (its covers edges repointed to the new digest) — cold-digest-spec §5.1.
+const (
+	StateDigestActive     = "active"
+	StateDigestSuperseded = "superseded"
 )
 
 // StateOrigin marks the task root fact (KindFact) seeded at task creation — the
@@ -22,4 +31,5 @@ const (
 	RelDerivedFrom = "derived_from"
 	RelYields      = "yields"
 	RelProves      = "proves"
+	RelCovers      = "covers" // digest --covers--> member (cold-digest-spec §1); source of truth for "which digest folds node X"
 )