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/src/index.ts

@@ -0,0 +1,75 @@
+/**
+ * arcrun WASM 零件 Worker (kbdb_upsert_block)
+ * POST / → JSON input → WASM (WASI preview1) → JSON output
+ * SDD: polaris/mira/.agents/specs/mira-app/design.md §3.5.12.4.1
+ *      matrix/arcrun/.agents/specs/arcrun/arcrun.md 三-B 新零件加入紀錄
+ */
+
+import componentWasm from '../component.wasm' assert { type: 'webassembly' };
+import { Hono } from 'hono';
+import { cors } from 'hono/cors';
+import { createWasiShim, type WasiHostFunctions } from '../../../cypher-executor/src/lib/wasi-shim';
+
+const app = new Hono();
+app.use('*', cors());
+
+app.get('/', (c) => c.json({ ok: true, component: 'kbdb_upsert_block' }));
+
+app.post('/', async (c) => {
+  let input: unknown;
+  try {
+    input = await c.req.json();
+  } catch {
+    return c.json({ success: false, error: 'request body must be JSON' }, 400);
+  }
+
+  try {
+    const result = await runWasm(input);
+    return c.json(result);
+  } catch (e) {
+    return c.json(
+      { success: false, error: e instanceof Error ? e.message : String(e) },
+      500,
+    );
+  }
+});
+
+export default app;
+
+async function runWasm(input: unknown): Promise<unknown> {
+  const hostFunctions: WasiHostFunctions = {
+    http_request: async (url, method, headersJson, body) => {
+      const headers: Record<string, string> = {};
+      if (headersJson) {
+        try {
+          const parsed = JSON.parse(headersJson);
+          if (parsed && typeof parsed === 'object') {
+            for (const [k, v] of Object.entries(parsed as Record<string, unknown>)) {
+              if (typeof v === 'string') headers[k] = v;
+            }
+          }
+        } catch {}
+      }
+      const init: RequestInit = { method, headers };
+      if (body && method.toUpperCase() !== 'GET' && method.toUpperCase() !== 'HEAD') {
+        init.body = body;
+      }
+      const res = await fetch(url, init);
+      return await res.text();
+    },
+  };
+
+  const shim = createWasiShim(JSON.stringify(input), hostFunctions);
+  const instance = await WebAssembly.instantiate(
+    componentWasm as WebAssembly.Module,
+    shim.imports,
+  );
+  shim.setMemory(instance.exports.memory as WebAssembly.Memory);
+  await shim.run(instance);
+
+  const stdout = shim.getStdout().trim();
+  const stderr = shim.getStderr().trim();
+  if (stderr) console.error('[kbdb_upsert_block wasm stderr]', stderr);
+  if (!stdout) throw new Error('WASM component produced no output');
+  return JSON.parse(stdout);
+}