feat(embed,ui04): E01 v2 contract skeleton — displayMode/scrollSpeed/reactionsEnabled, v2 loader entry and fixed 195x280 shell

- contracts: EmbedSlotSettings optional v2 fields (carousel default) + SlotSettingsPayload; gen + redocly green
- api: DTO validation + normalization full-set (cn manage + ov public projection share normalizeStoredSettings)
- embed-loader: params v2 parse/merge/url family, loader-v2.js fixed shell (border/radius/#10161E/safe-area, resize ignored, stack gap 12), dual-artifact build+sync; v1 source frozen
- viewer: EmbedPage v=2 branch, EmbedStream (viewport/track/real anchors/collapse/empty), stream.ts pure fns, i18n zh/en
- admin: types + utils (create default vertical-stream, clampScrollSpeed, v1/v2 snippet selection)
- tests: loader 36 (+9 v2), viewer 222, admin 82, api unit 51, v1 embed e2e 20, v2 E01 gate 2 (195x280 desktop+mobile cross-origin, no footer)

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

apps/viewer/src/__tests__/embed-feed.spec.ts

@@ -54,6 +54,10 @@ function makeSettings(overrides: Partial<EmbedSlotSettings> = {}): EmbedSlotSett
     offsetX: 16,
     offsetBottom: 24,
     zIndex: 1000,
+    // UI-04 v2 字段（响应恒为归一化全集；旧记录读取归一为 carousel）。
+    displayMode: 'carousel',
+    scrollSpeed: 16,
+    reactionsEnabled: true,
     ...overrides,
   };
 }