feat(videogen): canonical video-generation surface + llama-swap client

New videogen/ contract package (ADR-0019): Request/Result/Model/Provider
with the imagegen conventions. Text-to-video and image-to-video are one
surface (Request.InitImage, nil = t2v) since hybrid checkpoints like
Wan 2.2 TI2V serve both from one model; Result carries a single clip.

provider/llamaswap gains VideoModel(id) targeting the blocking
POST {base}/v1/videos/sync (multipart, model-routed by the fork's new
video routes): vLLM-Omni parameter names, OpenAI-style input_reference
file part, optional fields stay off the wire so per-model launch-flag
defaults apply. CLAUDE.md package map picks up audio/ (missed in #12)
and videogen/.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AXQxVhXBw8PwFAtsVrXSmj

videogen/videogen_test.go

@@ -0,0 +1,49 @@
+package videogen
+
+import "testing"
+
+func TestApplyDoesNotMutateOriginal(t *testing.T) {
+	orig := Request{Prompt: "a cat"}
+	got := orig.Apply(
+		WithSize("1280x704"),
+		WithNumFrames(81),
+		WithFPS(16),
+		WithSteps(4),
+		WithGuidanceScale(1.0),
+		WithNegativePrompt("blurry"),
+		WithSeed(42),
+		WithInitImage(Image{MIME: "image/png", Data: []byte{1}}),
+	)
+
+	if orig.Size != "" || orig.NumFrames != 0 || orig.FPS != 0 || orig.Steps != nil ||
+		orig.GuidanceScale != nil || orig.NegativePrompt != "" || orig.Seed != nil || orig.InitImage != nil {
+		t.Fatalf("original mutated: %+v", orig)
+	}
+
+	if got.Size != "1280x704" || got.NumFrames != 81 || got.FPS != 16 {
+		t.Errorf("size/frames/fps = %q/%d/%d", got.Size, got.NumFrames, got.FPS)
+	}
+	if got.Steps == nil || *got.Steps != 4 {
+		t.Errorf("steps = %v", got.Steps)
+	}
+	if got.GuidanceScale == nil || *got.GuidanceScale != 1.0 {
+		t.Errorf("guidance = %v", got.GuidanceScale)
+	}
+	if got.NegativePrompt != "blurry" {
+		t.Errorf("negative = %q", got.NegativePrompt)
+	}
+	if got.Seed == nil || *got.Seed != 42 {
+		t.Errorf("seed = %v", got.Seed)
+	}
+	if got.InitImage == nil || got.InitImage.MIME != "image/png" {
+		t.Errorf("init image = %+v", got.InitImage)
+	}
+}
+
+func TestApplyModelOptions(t *testing.T) {
+	called := false
+	ApplyModelOptions([]ModelOption{func(*ModelConfig) { called = true }})
+	if !called {
+		t.Fatal("model option not applied")
+	}
+}