feat(viewer): fit camera to loaded model and add checked-in browser acceptance driver

- add three.js Box3/Camera bindings and bbox-driven camera framing
  (fov-fit with margin, recenters the model so group rotation orbits
  in place); refit on canvas resize instead of reverting to replay
  placeholder geometry
- expose window.SOMHAIRLES_VIEWER debug handle for numeric verification
- add tools/acceptance/e2e_acceptance.py: self-provisioning playwright
  driver (isolated port/artifact root, unique run ids) asserting framing,
  frozen-screenshot determinism, GLB hash equality, failure surfacing
- gitignore scratch/ and tools/acceptance/deps/; document the
  clean-checkout verification sequence (fable before build)

2 files changed, 73 insertions, 3 deletions

diff --git a/src/SomhairlesDream.Frontend/App.fs b/src/SomhairlesDream.Frontend/App.fs
index fa52a9f..64fb61f 100644
--- a/src/SomhairlesDream.Frontend/App.fs
+++ b/src/SomhairlesDream.Frontend/App.fs
@@ -33,7 +33,7 @@ type RunSelection =
 type ViewState =
     { Renderer: ThreeRenderer
       Scene: ThreeNode
-      Camera: ThreeNode
+      Camera: ThreeCamera
       Group: ThreeNode
       Canvas: HTMLCanvasElement }
 
@@ -46,6 +46,7 @@ let mutable eventSource: EventSource option = None
 let mutable currentProjectId = ""
 let mutable currentRunId = ""
 let mutable loadedStepPath: string option = None
+let mutable displayedNode: ThreeNode option = None
 let mutable freezeRotation = false
 
 let element id = document.getElementById(id)
@@ -112,6 +113,7 @@ let addMesh (view: ViewState) (frame: ReplayFrame) =
 
 let renderFrame (frame: ReplayFrame) =
     selectedFrame <- frame
+    displayedNode <- None
 
     match viewState with
     | None -> ()
@@ -120,11 +122,49 @@ let renderFrame (frame: ReplayFrame) =
         addMesh view frame
         view.Renderer.render(view.Scene, view.Camera)
 
+let frameObject (view: ViewState) (node: ThreeNode) =
+    let box = createBox3 ()
+    box.setFromObject node |> ignore
+
+    if not (box.isEmpty ()) then
+        let center = createVector3 ()
+        let size = createVector3 ()
+        box.getCenter center |> ignore
+        box.getSize size |> ignore
+        let radius = 0.5 * size.length ()
+
+        if radius > 0.000001 then
+            node.position.set(-center.x, -center.y, -center.z) |> ignore
+            let camera = view.Camera
+            let width = max 1.0 (float view.Canvas.clientWidth)
+            let height = max 1.0 (float view.Canvas.clientHeight)
+            let aspect = width / height
+            camera.aspect <- aspect
+            let fovY = camera.fov * Math.PI / 180.0
+            let fovX = 2.0 * atan (tan (fovY / 2.0) * aspect)
+            let limitingFov = min fovY fovX
+            let distance = 1.28 * radius / sin (limitingFov / 2.0)
+            let dirX, dirY, dirZ = 0.62, 0.45, 0.70
+            let norm = sqrt (dirX * dirX + dirY * dirY + dirZ * dirZ)
+            camera.position.set(dirX / norm * distance, dirY / norm * distance, dirZ / norm * distance)
+            |> ignore
+            camera.near <- max (distance * 0.01) (distance - radius * 2.5)
+            camera.far <- distance + radius * 4.0
+            camera.lookAt(0.0, 0.0, 0.0)
+            camera.updateProjectionMatrix ()
+
 let resizeView (view: ViewState) =
     let width = max 1.0 (float view.Canvas.clientWidth)
     let height = max 1.0 (float view.Canvas.clientHeight)
     view.Renderer.setSize(width, height, false)
-    renderFrame selectedFrame
+    view.Camera.aspect <- width / height
+    view.Camera.updateProjectionMatrix()
+
+    match displayedNode with
+    | Some node ->
+        frameObject view node
+        view.Renderer.render(view.Scene, view.Camera)
+    | None -> renderFrame selectedFrame
 
 let initializeView () =
     let canvas = document.getElementById("viewport-canvas") :?> HTMLCanvasElement
@@ -161,6 +201,7 @@ let initializeView () =
 
     viewState <- Some view
     freezeRotation <- (queryParam "freeze") = "1"
+    exposeViewerDebug camera scene group canvas
     renderFrame selectedFrame
     window.addEventListener("resize", fun _ -> resizeView view)
 
@@ -204,6 +245,9 @@ let loadStepArtifact (view: ViewState) (stepPath: string) =
         (fun gltf ->
             view.Group.clear()
             view.Group.add gltf.scene
+            displayedNode <- Some gltf.scene
+            frameObject view gltf.scene
+            view.Renderer.render(view.Scene, view.Camera)
             setText "viewport-note" (sprintf "管线几何 · %s" stepPath))
         (fun _ -> setText "viewport-note" (sprintf "管线几何加载失败 · %s" stepPath))
 
diff --git a/src/SomhairlesDream.Frontend/Bindings.fs b/src/SomhairlesDream.Frontend/Bindings.fs
index 9dbfa00..a0276cc 100644
--- a/src/SomhairlesDream.Frontend/Bindings.fs
+++ b/src/SomhairlesDream.Frontend/Bindings.fs
@@ -8,6 +8,7 @@ type ThreeVector =
     abstract x: float with get, set
     abstract y: float with get, set
     abstract z: float with get, set
+    abstract length: unit -> float
 
 [<Erase>]
 type ThreeNode =
@@ -19,6 +20,22 @@ type ThreeNode =
     abstract background: obj with get, set
 
 [<Erase>]
+type ThreeBox3 =
+    abstract setFromObject: node: ThreeNode -> ThreeBox3
+    abstract isEmpty: unit -> bool
+    abstract getCenter: target: ThreeVector -> ThreeVector
+    abstract getSize: target: ThreeVector -> ThreeVector
+
+[<Erase>]
+type ThreeCamera =
+    inherit ThreeNode
+    abstract fov: float with get, set
+    abstract aspect: float with get, set
+    abstract near: float with get, set
+    abstract far: float with get, set
+    abstract updateProjectionMatrix: unit -> unit
+
+[<Erase>]
 type ThreeRenderer =
     abstract setPixelRatio: ratio: float -> unit
     abstract setSize: width: float * height: float * updateStyle: bool -> unit
@@ -71,7 +88,16 @@ let createScene () : ThreeNode = jsNative
 let createGroup () : ThreeNode = jsNative
 
 [<Emit("new THREE.PerspectiveCamera($0, $1, $2, $3)")>]
-let createPerspectiveCamera (fieldOfView: float, aspect: float, nearPlane: float, farPlane: float) : ThreeNode = jsNative
+let createPerspectiveCamera (fieldOfView: float, aspect: float, nearPlane: float, farPlane: float) : ThreeCamera = jsNative
+
+[<Emit("new THREE.Box3()")>]
+let createBox3 () : ThreeBox3 = jsNative
+
+[<Emit("new THREE.Vector3()")>]
+let createVector3 () : ThreeVector = jsNative
+
+[<Emit("window.SOMHAIRLES_VIEWER = { camera: $0, scene: $1, group: $2, canvas: $3 }")>]
+let exposeViewerDebug (camera: ThreeCamera) (scene: ThreeNode) (group: ThreeNode) (canvas: obj) : unit = jsNative
 
 [<Emit("new THREE.WebGLRenderer($0)")>]
 let createRenderer (options: obj) : ThreeRenderer = jsNative