feat(core): CORRECT004 — unmutated $state → const

.changeset/correct004-unmutated-state.md

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+---
+'@svelte-vitals/core': minor
+'svelte-vitals': minor
+'@svelte-vitals/mcp': minor
+---
+
+Add **CORRECT004 (unmutated $state)** — a Correctness/reactivity rule from #69.
+Flags a `let x = $state(...)` that is never written or escaped anywhere in the
+component (no reassignment, member/method mutation, bind, call-arg, or
+component-prop pass), so its reactivity is unused — use `const` (or `$state.raw`
+if only reassigned wholesale). Reported under `correctness` (info). `ComponentFacts`
+gains `constableStates`.

docs/src/content/docs/ja/rules/correct004.md

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+---
+title: CORRECT004 · 変更されない $state
+description: 変更されない $state には const（または $state.raw）を使います。
+---
+
+**重大度:** info · **カテゴリ:** correctness
+
+## チェック内容
+
+コンポーネント内のどこでも書き込み・エスケープされない `let x = $state(...)` を検出します — 再代入なし、変更なし（`x.a = …`、`x.push()`）、バインドなし（`bind:value={x}`）、関数やコンポーネントへの受け渡しなしのものです。コンポーネントのスクリプトとテンプレートを静的(CLI)解析します。
+
+## なぜ重要か
+
+変更されない `$state` は、使わないリアクティビティ（deep proxy と依存追跡）のコストを払っています。`const` の方が明確で軽量です。値をまるごと差し替えるだけ（プロパティは変更しない）なら `$state.raw` が適します。
+
+## 修正方法
+
+```svelte
+<script>
+  // let title = $state('Dashboard'); の代わりに
+  const title = 'Dashboard';
+
+  // まるごと差し替えるが deep mutate しないなら $state.raw:
+  let data = $state.raw(initial);
+  data = nextValue;
+</script>
+```

docs/src/content/docs/rules/correct004.md

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+---
+title: CORRECT004 · Unmutated $state
+description: Use const (or $state.raw) for a $state that is never mutated.
+---
+
+**Severity:** info · **Category:** correctness
+
+## What it checks
+
+Flags a `let x = $state(...)` whose value is never written or escaped anywhere in the component — not reassigned, not mutated (`x.a = …`, `x.push()`), not bound (`bind:value={x}`), not passed to a function or component. Checked by static (CLI) analysis of the component script and template.
+
+## Why it matters
+
+A `$state` that is never mutated pays for reactivity — deep proxying and dependency tracking — that it never uses. `const` is clearer and cheaper; `$state.raw` fits when you only ever reassign the value wholesale (never mutate its properties).
+
+## How to fix
+
+```svelte
+<script>
+  // Instead of: let title = $state('Dashboard');
+  const title = 'Dashboard';
+
+  // Reassigned wholesale but never deeply mutated? Use $state.raw:
+  let data = $state.raw(initial);
+  data = nextValue;
+</script>
+```