diff --git a/docs/superpowers/specs/2026-06-09-bundle-and-tests-design.md b/docs/superpowers/specs/2026-06-09-bundle-and-tests-design.md
new file mode 100644
index 0000000..3c1e62a
--- /dev/null
+++ b/docs/superpowers/specs/2026-06-09-bundle-and-tests-design.md
@@ -0,0 +1,111 @@
+# Bundle Vendor-Split + Test-Gap Fills — Design
+
+**Date:** 2026-06-09
+**Status:** Approved (brainstorming) — ready for implementation planning.
+**Issue:** #67 (vendor-split the bundle for cache stability/headroom; fill the audit's unit-test + story gaps).
+
+## Context
+
+The production build emits a single ~216 KB-gz entry chunk (the largest, measured by
+`scripts/check-bundle-size.mjs`, budget 250 KB) that bundles the framework deps (react-dom, react-router 7,
+@tanstack/react-query, @base-ui/react, i18next) together with app code — so every app edit invalidates the
+whole chunk's cache. `vite.config.ts` has no `build.rollupOptions`. Separately, four well-isolated units
+have no direct tests, and the composed combobox primitive has no story.
+
+All changes are additive/low-risk; `check:size`/`check:colors`/the existing tests are the guards.
+
+## Components
+
+### 1. Vendor split — `vite.config.ts`
+Add a top-level `build` block (sibling of `plugins`/`resolve`/`server`/`test`) with `manualChunks`:
+```ts
+build: {
+  rollupOptions: {
+    output: {
+      manualChunks: {
+        react: ["react", "react-dom", "react-router", "react-router-dom"],
+        "base-ui": ["@base-ui/react"],
+        query: ["@tanstack/react-query"],
+        i18n: ["i18next", "react-i18next"],
+      },
+    },
+  },
+},
+```
+- The app entry chunk then carries only app code; the listed deps land in cache-stable vendor chunks
+  (`react-*.js`, `base-ui-*.js`, `query-*.js`, `i18n-*.js`). `react-router` **and** `react-router-dom` are
+  both listed so router code (v7 re-exports `react-router` internally) stays in one chunk and React isn't
+  duplicated.
+- Only affects `pnpm build` (production); the Storybook/vitest browser builder is independent, so the
+  test suite is unaffected.
+- `check:size` measures the largest emitted chunk; after the split the largest is a vendor chunk well under
+  the 250 KB budget (verified in implementation by running `pnpm build` + `pnpm check:size` and reporting
+  the chunk sizes). If a chunk is unexpectedly oversized or React duplicates, adjust the groups.
+
+### 2. Test-gap fills (4 unit tests)
+
+**`objects/prune-fields.test.ts`** (new) — `pruneFields(fields, localizedTextKeys: Set<string>, defaultLang)`:
+- scalars pass through; `undefined`/`null`/`""` top-level values are dropped.
+- a non-array object value whose key is in `localizedTextKeys` keeps **only** the `defaultLang` entry;
+  other-language entries are dropped.
+- a non-array object value whose key is **not** in `localizedTextKeys` keeps all (non-empty) language
+  entries.
+- empty inner entries (`""`/`null`/`undefined`) are filtered, and a map left with zero entries is dropped
+  entirely.
+
+**`components/delete-confirm-dialog.test.tsx`** (new) — the delete-in-use flow:
+- render `<DeleteConfirmDialog description=… onConfirm=… />`, open it (click the trigger), click the
+  confirm action.
+- when `onConfirm` rejects with `InUseError(3)`, the dialog shows the `actions.inUse` message (containing
+  the count `3`) and **stays open** (the confirm action is still present / the dialog isn't dismissed).
+- when `onConfirm` resolves, the dialog closes.
+(`DeleteConfirmDialog` routes its catch through `errorMessageKey`, which maps `InUseError` →
+`actions.inUse` with the count — so this also exercises the shared error mapping.)
+
+**`lib/labels.test.ts`** (new) — `labelText(labels, lang)`:
+- exact `lang` match wins; else falls back to the `"en"` label; else the first label; else `""` for an
+  empty array.
+
+**`lib/format-date.test.ts`** (new) — `formatDate(value, lang)`:
+- a valid `"YYYY-MM-DD"` formats via `Intl.DateTimeFormat(lang, { dateStyle: "medium" })` and is **not**
+  shifted off its calendar day (assert the rendered string contains the expected day number for a fixed
+  date + a fixed `lang` like `"en"`).
+- `null` → `"—"`; a non-date string (e.g. `"not-a-date"`) is returned unchanged; a non-string non-null
+  (e.g. a number) is `String()`-ified.
+
+### 3. Combobox story — `components/ui/combobox.stories.tsx` (new)
+A visual/interactive story for the composed combobox primitive, mirroring the existing `ui/*` story format
+(`@storybook/react-vite` `Meta`/`StoryObj`, single-quote + no-semicolon, `tags: ['ai-generated']`, a `play`
+using `canvas` + `storybook/test`). A small controlled wrapper renders `ComboboxRoot` with a handful of
+string options (`ComboboxInput`/`ComboboxClear`/`ComboboxTrigger`/`ComboboxPopup`/`ComboboxList`/
+`ComboboxItem`/`ComboboxEmpty`), matching how `OptionsCombobox` composes them. A `Default` story; the
+`play` asserts the input renders and (optionally) typing filters the list. (Runs as a browser test in the
+Storybook vitest project — a few seconds in CI.)
+
+## Error handling / edges
+- `manualChunks`: Rollup orders chunks by the import graph, so vendor chunks load before the app chunk that
+  imports them — no load-order/duplicate-React risk when react+react-dom+router share one named chunk.
+- `formatDate` parses `\`${value}T00:00:00\`` (local midnight) to avoid a UTC day-shift; the test pins a
+  fixed `lang` to keep `Intl` output deterministic (assert a substring like the day number, not the full
+  locale string, to stay robust across ICU versions).
+- `delete-confirm-dialog` test drives the Base UI AlertDialog in its portal (`within(document.body)` for the
+  confirm action), mirroring existing portal-aware tests.
+
+## Testing
+- The 4 unit tests run in the jsdom project; the story adds one Storybook browser test.
+- **Gate:** `typecheck`/`lint`/`test`/`build`/`check:size`/`check:colors` green; `check:size` reports the
+  new largest (vendor) chunk under budget; no new dependency; no new i18n keys; no codename. en/sv parity
+  unaffected.
+
+## Acceptance criteria
+1. `vite.config.ts` has `build.rollupOptions.output.manualChunks` splitting react/base-ui/query/i18n into
+   their own chunks; `pnpm build` succeeds and `check:size` passes (largest chunk reported, < 250 KB gz, no
+   React duplication).
+2. New tests exist and pass: `prune-fields.test.ts`, `delete-confirm-dialog.test.tsx` (InUseError →
+   `actions.inUse`, stays open), `labels.test.ts`, `format-date.test.ts`.
+3. `components/ui/combobox.stories.tsx` exists with a working `Default` story (browser-test green).
+4. Full gate green; no new dependency; no new i18n keys; no codename; existing tests unchanged.
+
+## Out of scope → follow-ups
+- The buildkit/Dockerfile CI migration (the robust fix for the slow native runner; overlaps #25).
+- Deeper treeshaking/bundle analysis; route-level code-split changes beyond the existing `lazy()` boundaries.