docs: add advanced skills for Vitest, Pinia, and Vue built-ins

Add comprehensive reference documentation for:
- Vitest: environments, projects/workspaces, type testing, vi utilities
- Pinia: HMR, Nuxt integration, SSR setup
- Vue: built-in components (Transition, Teleport, Suspense, KeepAlive) and advanced directives

.agents/skills/vueuse-functions/references/useProjection.md

@@ -0,0 +1,38 @@
+---
+category: '@Math'
+related: createGenericProjection
+---
+
+# useProjection
+
+Reactive numeric projection from one domain to another.
+
+## Usage
+
+```ts
+import { useProjection } from '@vueuse/math'
+
+const input = ref(0)
+const projected = useProjection(input, [0, 10], [0, 100])
+
+input.value = 5 // projected.value === 50
+input.value = 10 // projected.value === 100
+```
+
+## Type Declarations
+
+```ts
+/**
+ * Reactive numeric projection from one domain to another.
+ *
+ * @see https://vueuse.org/useProjection
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useProjection(
+  input: MaybeRefOrGetter<number>,
+  fromDomain: MaybeRefOrGetter<readonly [number, number]>,
+  toDomain: MaybeRefOrGetter<readonly [number, number]>,
+  projector?: ProjectorFunction<number, number>,
+): ComputedRef<number>
+```