feat: add image/texture infrastructure for visualization

[mtao]

Summary

• Add complete image/texture infrastructure to balsa's visualization subsystem, implementing Phase 1 and Phase 2 of the art-viewer plan
• Introduces VulkanTexture (RAII GPU texture with full and partial upload), ImageData (CPU pixel buffer scene graph feature with version-tracked dirty regions), ImagePipelineManager (Vulkan pipeline with fullscreen triangle, descriptor sets, per-frame UBOs), VulkanImageDrawable (syncs ImageData to GPU texture per frame), and ImageScene (orthographic 2D viewer with pan/zoom/fit)
• Adds HDR tone mapping shaders (exposure/gamma in fragment shader), ImGui controls panel, PPM I/O (P6 binary), and a standalone image_viewer_glfw tool with scroll-zoom and middle-drag pan

New Files (21 files, ~3000 LOC)

Headers

• visualization/include/balsa/visualization/vulkan/texture.hpp — VulkanTexture RAII wrapper
• visualization/include/balsa/scene_graph/ImageData.hpp — CPU pixel buffer feature (RGBA8/RGBAF32, dirty tracking)
• visualization/include/balsa/visualization/vulkan/image_pipeline.hpp — Pipeline manager + UBO structs
• visualization/include/balsa/visualization/vulkan/vulkan_image_drawable.hpp — Per-frame GPU sync drawable
• visualization/include/balsa/visualization/vulkan/image_scene.hpp — Orthographic 2D image scene
• visualization/include/balsa/visualization/vulkan/imgui/image_controls_panel.hpp — ImGui controls
• visualization/include/balsa/visualization/image_io.hpp — PPM I/O declarations

Implementations

• visualization/src/vulkan/texture.cpp — Full VulkanTexture (create, upload, update_region, layout transitions)
• visualization/src/scene_graph/ImageData.cpp — Pixel buffer management with dirty region merging
• visualization/src/vulkan/image_pipeline.cpp — Descriptor layout, pool, pipeline creation, shader compilation
• visualization/src/vulkan/vulkan_image_drawable.cpp — Version-tracked sync, partial texture updates
• visualization/src/vulkan/image_scene.cpp — Scene lifecycle, orthographic projection, pan/zoom
• visualization/src/vulkan/imgui/image_controls_panel.cpp — Exposure/gamma/channel/zoom/pan controls
• visualization/src/image_io.cpp — PPM P6 reader/writer (RGB→RGBA expansion)

Shaders

• visualization/resources/glsl/image.vert — Fullscreen triangle via gl_VertexIndex
• visualization/resources/glsl/image.frag — HDR tone mapping + channel isolation

Tools

• visualization/tools/image_viewer_glfw.cpp — Standalone viewer with CLI11, ImGui overlay

Tests

• 15 new test cases in test_scene_graph.cpp covering ImageData (creation, RGBA8/RGBAF32 pixels, version tracking, partial updates, dirty region merging, display parameters, error cases) and PPM I/O (round-trip, error handling)

Motivation

This lays the foundation for:

1. ART ray tracer live preview — progressive rendering with partial texture updates
2. MultiscreenManga comic reader port — image display with pan/zoom

Testing

All 8 balsa test suites pass (44 scene_graph test cases, 312 assertions total).