feat: 3D rendering with OBJ support and Utah Teapot example

## Goal

Extend the engine from 2D to 3D rendering. Add OBJ mesh loading, a depth buffer, a 3D camera with a perspective projection, and demonstrate it with the classic Utah Teapot.

## Engine work

### Depth buffer
- Add depth image + `VkImageView` to the renderer (`spock.renderer.vulkan`)
- Enable depth testing in the pipeline builder (`VkPipelineDepthStencilStateCreateInfo`)
- Add `pipeline/depth-test` option: `(pipeline/depth-test cfg true)`

### Camera / UBO
- `spock.camera` — perspective camera record: position, target, fov, near, far
- `camera->matrix` — returns a 4×4 view-projection matrix (column-major, Vulkan NDC)
- Upload per-frame via a uniform buffer object (UBO) or push constants (64-byte push constant block fits a mat4 + model mat4 as two separate draws)

### Vertex format
- 3D vertices: `vec3 position`, `vec3 normal`, `vec2 uv` — 32 bytes per vertex
- New `VkVertexInputAttributeDescription` entries in pipeline builder

### OBJ loader — `spock.obj`
- Parse Wavefront OBJ: `v` (vertices), `vn` (normals), `vt` (UVs), `f` (faces, triangulate quads)
- Returns `{:vertices FloatBuffer :indices IntBuffer :vertex-count int}`
- Index buffer support in renderer (currently only vertex buffers)

### `:mesh` renderable type (spock.edn)
```edn
{:type :mesh
:file "examples/teapot/teapot.obj"
:color [0.8 0.6 0.3 1.0]}
```
- Loads OBJ, uploads vertex + index buffers
- Per-instance push constants: 4×4 model matrix + color

### Shaders
- `src/shaders/mesh.vert` — `vec3 pos`, `vec3 normal` in; push constant mat4 model + mat4 view-proj; Phong lighting
- `src/shaders/mesh.frag` — simple diffuse + ambient

## Utah Teapot example
```
examples/teapot/
game.edn
settings.edn
teapot.obj ← Newell teapot OBJ (public domain)
scenes/teapot.edn
scripts/teapot.clj ← rotate teapot over time, orbit camera
```

Makefile targets: `shaders/mesh`, `run/teapot`, `teapot`

## Depends on
- #49 (image loading) — optional, needed for textured meshes; mesh can ship with flat color first
- Depth buffer is a prerequisite for correct 3D rendering

Start by reading spock.renderer.vulkan, the pipeline builder, spock.camera, and spock.obj to map the existing renderer and buffer abstractions. Then inspect src/shaders/mesh.vert, src/shaders/mesh.frag, and the examples/teapot files. Done means the depth-tested 3D mesh path, OBJ support, shaders, and teapot targets work together as described.

Written by the indexing model from the issue text.