docs(task-31): graph node merge & background suggestion task spec v1 (#1931)

task #31 spec v1 lock — graph 节点合并扫描 + 后台建议任务设计文档入仓。

## 设计核心

- **scope reframe**: extract / fix / extend Wave 7 §K.12.4 全栈，不 build new
- **独立 queue family** `q:graph_curation_run`：lane 不污染 Modality + DocumentIndex + reconciler，独立 push/pop API
- **trigger 三策略 reconcile**: manual/cron full sweep 走 worker pop → generate_graph_curation_run_task；auto_post_ingest 保 sync inline detect_for_sync 但同 description-free invariant
- **复用 GraphCurationSuggestion table**：不引入新 merge_suggestion table，仅 extend 4 新 status enum + evidence_refs field
- **状态机 Option B (apply_pending + ACCEPTED legacy)**: pending → dismissed | rejected | apply_pending → applying → applied | apply_failed；现有 ACCEPTED 历史 sync handle_action terminal status 保留 legacy read-only，新 async path zero-write gate
- **description-free 6 call sites + 1 apply path** (Wave 5 invariant): candidate_generation.py:43/179-181/196-197 + dto.py:59-65/101-105 + merge_candidate_detector.py:257-284 + :322-328 + lineage_merge.py:246-317 apply variant
- **LineageEntityMerger application-layer cross-backend contract** (Protocol 不含 merge_entities，复用 LineageGraphStore primitives)
- **entity_type scope lock 三层**: v1 仅 compatibility/penalty signal，suggestion 容忍 type 近似展示 observed_types/type_conflict/suggested_entity_type，entity_type_alias 独立 suggestion kind 移 Phase B/P1 follow-up #31-C3
- **复用 /graphs/merge-suggestions endpoint + extend SUGGESTION_ACTIONS dismiss + Pydantic Field validator confidence_score [0,1]**

## 集体 8/8 lane LGTM 收齐

- @bryce (msg=9e49d440): 5 BLOCKER 全清 + entity_type scope lock + Migration chain 一致性
- @weston (msg=ed202960 + 92dd89ff): 五类 consistency sweep + entity_type 三层架构 + Migration chain
- @huangzhangshu (msg=9a4cbd61 + 68783841): 五类旧口径清成 Phase A/B gate + enum count micro-fix
- @ziang (msg=760b7341 + 0b761117): impl-lane 5 BLOCKER + state machine Option B + enum count
- @huangheng (msg=535de81b): Lesson framework v5/v6/v7/v8/v9/v13/v14/v16/v17 + Lesson #18 候选 cross-link + Migration chain 时序 全一致
- @dongdong (msg=8316b45a): FE/UI scoped + entity_type FE 友好性 + state machine
- @Planetegg (msg=7d428e33): SRE/deploy Helm render gate symbolic lane assertion
- @cuiwenbo (msg=594fbd4f): 3 NIT (endpoint reuse + status enum FE typed schema sync + confidence_score [0,1] validator) 全 fold

## CI 状态

- lint-and-unit ✅
- e2e-http-smoke 3/3 ✅  
- e2e-http-provider-preflight 3/3 ✅
- docs-only lite gate 满足

## 关联

- 不阻塞 PR #1932 (huangheng sediment merged dc79aad) / PR #1933 (chenyexuan merged 1024ef9) / task #61 P1/P2 follow-up / task #11 GC orphan vector follow-up
- Phase A 4 sub-task 派单 spec lock 后立即可启动 (推荐 owner: A1+A3 Bryce/ziang / A2 ziang / A4 dongdong+cuiwenbo)

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Author: earayu