Redesign the Storage Monitor as a storage-composition breakdown (snippets · datasets · app)

docs/architecture/10-interaction-and-feedback.md

@@ -263,6 +263,31 @@ relative date and size), in a fixed-width leading slot so it never shifts adjace
 _(Consulted via /council → GOV.UK Tag, Carbon status-indicator-pattern, APG. This bullet is the
 contract; cite it, not the external source.)_
 
+**Resolved — storage composition indicator.** The library-footer Storage Monitor (spec §02)
+shows what storage is **made of** — Snippets · Datasets · App — as a proportional bar plus a
+labelled legend, **not** a "used of quota" gauge. The browser quota is a padded, unreliable
+approximation (web.dev → _storage-for-the-web_), so a budget fraction is false precision; we
+show real measured sizes instead.
+
+- **Not a meter.** No `role="meter"`/`progressbar`: a meter needs a meaningful maximum, and a
+  composition with no trustworthy ceiling has none (APG → `meter`: _"should not be used to
+  represent a value … [without] a meaningful maximum"_). The visual bar is **decorative**
+  (`aria-hidden`); the **legend's text labels + sizes are the accessible source of truth**, so
+  meaning never rests on hue (WCAG 1.4.1).
+- **Part-to-whole in a tiny space.** A single proportional stacked bar suits a **few** segments
+  (we have three) — FT Visual Vocabulary (Part-to-whole) + Datawrapper (stacked bar for "a few
+  shares"; bar "when precise reading matters") — paired with absolute byte labels for the precise read.
+- **Unavailable degrades, not disappears.** Snippets + datasets are measured from our own data, so
+  they always show; only the **App** segment (which needs the origin estimate) drops out when the
+  Storage Manager API is absent.
+- **No proactive "almost full" warning.** Dropping the old 0.8/0.95 thresholds is intentional —
+  they keyed off the untrustworthy quota, and a fake fuel gauge fails NN/g #1 (_visibility of system
+  status_) more than it serves it. The genuine out-of-room event surfaces at **save time** as an
+  actionable error (`services/storage-errors.ts` → recover by deleting), satisfying NN/g #9.
+
+_(Consulted via /council → WAI-ARIA APG `meter`, FT Visual Vocabulary + Datawrapper (part-to-whole),
+web.dev storage, NN/g #1/#9, WCAG 1.4.1. This bullet is the contract; cite it, not the sources.)_
+
 **Resolved — library search (Carbon active-search).** The snippet-library search (spec
 §02) is an **unlabelled active-search input** pinned above the list: `type="search"` with a
 leading magnifier and `aria-label="Search snippets"` (no visible label — the icon +