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/tsconfig.json

@@ -0,0 +1,11 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "ES2022",
+    "moduleResolution": "bundler",
+    "lib": ["ES2022"],
+    "types": ["@cloudflare/workers-types"],
+    "strict": true,
+    "noEmit": true
+  }
+}