Improve viewer navigation, Vulkan setup, and tests

Add richer viewer navigation state (scroll, norm/max scroll, zoom pivot, pending/frame counters) and helpers to reset/sync scroll and queue zoom pivots. Rework viewport/layout logic with compute_image_viewport_layout, fit/recenter/fit-zoom calculation, and smooth application of pending zoom pivots. Introduce app_shortcut wrapper for global shortcuts and wire many existing shortcuts to it; add a Re-center action and small UI tweaks. Refactor Vulkan initialization: choose instance API version, split instance/device setup, select physical device and queue with scoring and extension checks, handle portability subset and improve surface/window cleanup. Add nearest-mag sampler and descriptor set for improved sampling views. Add test-engine enhancements: parse key chords from env, read key chord env var, add state-dump test to write viewer JSON, and other automation/test helpers. Misc: remove unused linear_interpolation push-constant usage and adjust some layout/padding values.

Author: ssh4net