Add storage-estimate core and adapter (M6 Storage Monitor groundwork)

Portable summarizer (clamped fraction, ok/warning/critical level, availability
flag, humanizeBytes) with the browser StorageManager.estimate() feature-detect
isolated to the infrastructure adapter. No UI consumer yet — core-first per the
AI developer protocol; the Storage Monitor panel (spec §02) wires to it next.

src/app/infrastructure/storage-estimate.ts

@@ -0,0 +1,39 @@
+/**
+ * Storage estimate adapter (spec §02 → Storage Monitor; docs/architecture/02 §6).
+ *
+ * This is the ONLY module that touches the browser's Storage Manager API
+ * (`navigator.storage.estimate()`). It feature-detects the API, then defers all
+ * presentation math to the portable core (`src/core/storage-estimate.ts`). When
+ * the API is unavailable — older browsers, insecure contexts — it returns an
+ * unavailable summary rather than throwing, so callers get a total function.
+ */
+
+import { summarizeStorage, type StorageSummary } from '../../core/storage-estimate';
+
+/** The unavailable summary, returned when the Storage Manager API is absent. */
+const UNAVAILABLE: StorageSummary = {
+  usedBytes: 0,
+  quotaBytes: 0,
+  fraction: 0,
+  level: 'ok',
+  available: false,
+};
+
+/**
+ * Read the browser's storage estimate and summarize it. Returns an unavailable
+ * summary when `navigator.storage.estimate` is missing or the call rejects;
+ * never throws. All math lives in `summarizeStorage`.
+ */
+export async function readStorageEstimate(): Promise<StorageSummary> {
+  const storage = typeof navigator !== 'undefined' ? navigator.storage : undefined;
+  if (!storage || typeof storage.estimate !== 'function') {
+    return UNAVAILABLE;
+  }
+
+  try {
+    const { usage, quota } = await storage.estimate();
+    return summarizeStorage({ usage, quota });
+  } catch {
+    return UNAVAILABLE;
+  }
+}