port ujsx from Deno-only to cross-platform (Node/Bun/Deno)

Add npm project configuration (package.json, tsconfig.json, tsup, vitest)
matching the taskgraph_ts conventions. All source imports changed from .ts
to .js extensions for Node16 module resolution. Tests migrated from Deno.test
to vitest. Fixed strict type errors (noUncheckedIndexedAccess). Preserved
deno.json with sloppy-imports for dual Deno/Node compatibility.

Subpath exports: schema, h, reactive, context, events, pointer, host,
transform, jsx-runtime — plus barrel export at root.

Build: ESM + CJS dual output via tsup. 22 tests passing.

deno.json

@@ -0,0 +1,32 @@
+{
+  "name": "@alkdev/ujsx",
+  "version": "0.1.0",
+  "exports": {
+    ".": "./src/mod.ts",
+    "./core": "./src/mod.ts",
+    "./schema": "./src/core/schema.ts",
+    "./h": "./src/core/h.ts",
+    "./reactive": "./src/core/reactive.ts",
+    "./context": "./src/core/context.ts",
+    "./events": "./src/core/events.ts",
+    "./pointer": "./src/core/pointer.ts",
+    "./host": "./src/host/config.ts",
+    "./transform": "./src/transform/registry.ts",
+    "./jsx-runtime": "./src/core/jsx-runtime.ts"
+  },
+  "tasks": {
+    "test": "npx vitest run",
+    "check": "deno check --sloppy-imports src/mod.ts"
+  },
+  "imports": {
+    "@alkdev/typebox": "npm:@alkdev/typebox",
+    "@alkdev/pubsub": "../../@alkdev/pubsub/src/index.ts",
+    "@preact/signals-core": "npm:@preact/signals-core",
+    "@std/assert": "jsr:@std/assert@1"
+  },
+  "compilerOptions": {
+    "jsx": "react-jsx",
+    "jsxImportSource": "@alkdev/ujsx"
+  },
+  "unstable": ["sloppy-imports"]
+}