add app test suite — unit, widget, golden, a11y (168 tests)

Four layers under app/test/:

  * kernel/, extension/ — unit tests for every kernel service and
    the extension contract. i18n fallback chain gets the full
    matrix (exact/lang/default/placeholder, namespace isolation,
    unknown namespace, interpolation). Theme resolver and
    extension-manager topo sort covered.
  * widgets/, builtin/ — widget tests for every primitive and each
    Tier 0 built-in. Assertions reach into the Semantics node so
    a missing label fails structurally, not visually.
  * goldens/ — Alchemist + Ahem font for cross-platform pixel
    stability; primitives only (button, tab bar, icon set). No
    goldens for compositions — they'd churn through every tier.
  * a11y/ — contract-level gate: semantic coverage walks the
    built-in catalogue, contrast walks every token pair in every
    bundled theme against WCAG-AA thresholds (catches real
    regressions: the first run caught `tab.inactive_text` at 2.81
    in summer-night), i18n coverage asserts every referenced key
    exists in its catalog (exists-in-map, not "key == value" — too
    ambiguous for keys that happen to equal their translation).

Helpers under helpers/: `KernelFixture.create()` boots a
KernelServices with in-memory stores + a FakeDaemonClient (no
socket, drivable connected-state), `widget_harness` wraps a
widget in the minimum tree that resolves theme + i18n.

Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -30,6 +30,12 @@ heading, and (b) bumping `project.yaml` `version:` in the same commit.
 
 ### Added
 
+- App-level test suite under `app/test/` — 168 tests across four layers:
+  - **Unit** (`kernel/`, `extension/`) — events bus, settings (scope + YAML round-trip), log, i18n fallback chain matrix, theme resolver + loader + controller, panel registry + arrangement, command registry + keybinding parser + palette filter, extension-manager dep-order / cycle detection / enable-disable, manifest loader, extension scanner.
+  - **Widget** (`widgets/`, `builtin/`) — every primitive's Semantics presence + token consumption + hover/press states; each Tier 0 built-in's contributions, view, and locale-switch re-render.
+  - **Golden** (`goldens/`) — widget primitives only, Alchemist + Ahem font; PNG fixtures checked in under `_files/ci/` and `_files/linux/`.
+  - **A11y** (`a11y/`) — `semantic_coverage_test.dart` (contract-level check that every built-in carries title + version + label-ready contributions), `contrast_test.dart` (WCAG-AA ratio gate on every bundled theme's canonical token pairs), `i18n_coverage_test.dart` (asserts every Tier-0-referenced key is present in its `en_US` catalog), `keyboard_traversal_test.dart` (focusability smoke).
+- Test helpers under `app/test/helpers/` — `KernelFixture` (boots a KernelServices with in-memory defaults + a fake daemon for widget-level tests), `FakeDaemonClient` (subclasses the real client, no socket, drivable connected-state), `golden_harness` (Alchemist config with Ahem font for cross-platform pixel stability), `widget_harness` (wraps a widget in Directionality + ClideKernel + ClideTheme + MediaQuery).
 - Flutter desktop app scaffold under `app/` with a bare `WidgetsApp` root (no Material, no Cupertino) and the Tier 0 three-column layout.
   - **Kernel** (`app/lib/kernel/`) — 18 services consumed by every extension: `settings` (scope-resolved get/set across `app.*`/`project.*`/`ext.*`), `project`, `extensions`, `theme`, `panels` (slot registry + arrangement), `events`, `ipc`, `commands` (+ palette + keybinding resolver), `clipboard`, `files`, `notify`, `dialog` (single-at-a-time modal router), `tray`, `secrets`, `os`, `net`, `focus`, and `log`. Unified in a `ClideKernel` `InheritedWidget`.
   - **i18n** (`app/lib/kernel/src/i18n/`) — text-driven lookup ported from [fframe](https://github.com/postmeridiem/fframe)'s `L10n`: namespaced JSON catalogs, `string()` / `interpolated()` calls with caller-supplied placeholders, and a proper locale fallback chain (exact → language → default-country → default-language → placeholder). Improves on fframe's design by adding the chain, which fframe lacks.