feat(arcrun): add kbdb_upsert_block component for idempotent block writes

對應 mira 7B.3f：per-entity index-entry 維護需要「找有則 PATCH 沒找到 POST」，
arcrun workflow 沒 IF/branch 能力（已知限制 #1 + 新 P1 #1），用 kbdb_upsert_block
零件把分支邏輯封進零件內：GET /blocks?page_name=X → user_id filter → 找到 PATCH 沒找到 POST。
page_name 當 idempotency key，未來其他「找有則改沒則建」場景共用。

SDD：polaris/mira/.agents/specs/mira-app/design.md §3.5.12.4.1
     matrix/arcrun/.agents/specs/arcrun/arcrun.md 三-A P1 #1 + 三-B 新零件加入紀錄

.component-builds/kbdb_upsert_block/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "arcrun-kbdb-upsert-block",
+  "version": "1.0.0",
+  "private": true,
+  "type": "module",
+  "dependencies": {
+    "hono": "^4.7.0"
+  },
+  "devDependencies": {
+    "@cloudflare/workers-types": "^4.20250408.0",
+    "typescript": "^5.4.0",
+    "wrangler": "^4.0.0"
+  }
+}