feat(vision): image paste + transcript image rendering + vision badge

Vision & vision-handoff frontend (consumes the backend's additive
[email protected] / [email protected] image types — no version bump).

Contracts mirrored:
- .dispatch/wire.reference.md: ImageChunk added to the Chunk union +
  ImageChunk/ImageInput interfaces.
- .dispatch/transport-contract.reference.md: ChatRequest.images,
  ModelMetadata.vision, + ImageChunk/ImageInput re-exports.

Core (core/chunks):
- conformance: assertChunkExhaustive handles the new 'image' variant
  (the guard caught it — its purpose).
- appendUserMessage(state, text, images?) echoes a [text, image, ...]
  user run; the user-message event dedup scans the trailing user run
  (not just the last chunk) so an image-bearing echo doesn't duplicate
  the text; applyHistory's during-gen dedup matches a multi-chunk echo
  by content equality (chunkContentEquals + trailingRun helpers).

UI:
- ChatView renders user 'image' chunks as lazy <img> bubbles; a
  non-vision model's persisted [image, analysis-text] both render.
  read_image tool renders generically (no special-casing).
- Composer: clipboard paste / file picker / drag-drop of images ->
  base64 data URLs, thumbnail previews with remove, forwarded on
  chat.send (omitted when none). Image-only sends allowed; steering
  (chat.queue) never forwards images.
- ModelSelector: vision badge (isVisionModel) marks vision-capable
  models; indicator shows native-vision vs vision-handoff hint.

Store wiring: ChatStore.send + AppStore.send + App.svelte handleSend
thread images through; chat.send still omits cwd (only images added).

Verification: svelte-check 0/0; vitest 901/901 (run twice, +34 new);
biome clean; vite build OK. See backend-handoff.md §2j.

Not merged or pushed.

2 files changed, 41 insertions, 1 deletions

diff --git a/src/core/wire/conformance.test.ts b/src/core/wire/conformance.test.ts
index 880af07..0d955d5 100644
--- a/src/core/wire/conformance.test.ts
+++ b/src/core/wire/conformance.test.ts
@@ -128,9 +128,32 @@ describe("classifies every Chunk type", () => {
       },
       { type: "error" as const, message: "e" },
       { type: "system" as const, text: "s" },
+      { type: "image" as const, url: "data:image/png;base64,AAAA", mimeType: "image/png" },
     ];
     const labels = chunks.map(assertChunkExhaustive);
-    expect(labels).toEqual(["text", "thinking", "tool-call", "tool-result", "error", "system"]);
+    expect(labels).toEqual([
+      "text",
+      "thinking",
+      "tool-call",
+      "tool-result",
+      "error",
+      "system",
+      "image",
+    ]);
+  });
+
+  it("covers all 7 Chunk variants", () => {
+    // Keeps the exhaustive guard honest: a new Chunk.type variant must be added
+    // both here and to `assertChunkExhaustive` or the `satisfies never` errors.
+    expect([
+      "text",
+      "thinking",
+      "tool-call",
+      "tool-result",
+      "error",
+      "system",
+      "image",
+    ] as const).toHaveLength(7);
   });
 });
 
@@ -222,6 +245,21 @@ describe("ChatSendMessage shape is constructible", () => {
     expect(msg.model).toBe("default/gpt-4");
     expect(msg.cwd).toBe("/tmp");
   });
+
+  it("constructs a ChatSendMessage with pasted images", () => {
+    const msg: ChatSendMessage = {
+      type: "chat.send",
+      conversationId: "c1",
+      message: "what's in this image?",
+      images: [
+        { url: "data:image/png;base64,AAAA", mimeType: "image/png" },
+        { url: "https://example.com/cat.jpg" },
+      ],
+    };
+    expect(msg.images).toHaveLength(2);
+    expect(msg.images?.[0]?.mimeType).toBe("image/png");
+    expect(msg.images?.[1]?.mimeType).toBeUndefined();
+  });
 });
 
 describe("ConversationHistoryResponse shape is constructible", () => {
diff --git a/src/core/wire/conformance.ts b/src/core/wire/conformance.ts
index 412d80a..bfa67dc 100644
--- a/src/core/wire/conformance.ts
+++ b/src/core/wire/conformance.ts
@@ -60,6 +60,8 @@ export function assertChunkExhaustive(chunk: Chunk): string {
       return "error";
     case "system":
       return "system";
+    case "image":
+      return "image";
     default:
       return chunk satisfies never;
   }