feat: more robust automated test suite

[mcoliver]

Description

We need a more comprehensive and cross platform test suite especially around compiled builds on multiple operating systems and bundled cli executables. We need to cover all supported formats, file i/o, conversion, dependency resolution, UI, mac, win, and linux. Need to hook this into our pre and post build ci/cd to have better confidence around releases.

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Code of Conduct

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[mcoliver]

Progress update: the first pass landed in #64 (squash 999e8ba0).

Done:

• ctest runs on macOS, Windows and Linux in branch-build, dev-build and release, and blocks the build (the Windows step was previously continue-on-error).
• cli-smoke: every bundled CLI starts, resolves its runtime dependencies and exits correctly within a timeout. It uses the Windows .cmd wrappers and suppresses loader error dialogs. This covers the fix: utvpkg fails to find dependencies on Windows 11 #58 class of failure.
• image-format-roundtrip: every image format utvio -formats advertises as writable is written and read back, then checked for orientation, channels and values. This covers the fix: utvio fails to write PNG, DPX, WebP and QOI (blank "ERROR:", exit code 255) #59 class of failure.
• Headless CI support: utvio works on GPU-less macOS (software GL fallback) and Linux (Xvfb).
• Bugs found and fixed along the way: the media library thread race (an intermittent utvio abort), utvio exiting on GPU-less Macs, utvio segfaulting without an X display, and NV-only fence symbols on Mesa.

Remaining:

• UI automation: launch the app, load media, basic interaction, screenshots / pixel checks
• Post-package tests: the signed and notarized macOS app, the Homebrew cask, and the Windows installer on clean machines
• Movie and audio round-trips (ProRes, H.264/HEVC, DNx, audio tracks) through utvio
• Color pipeline / OCIO golden-image suite (shared with phase 1 of feat(render): port IPCore renderer from OpenGL to QRhi (Metal / Vulkan / D3D12) #70)
• Full Linux coverage. Blocked by fix(linux): utv* tools staged without launcher wrappers after rv→utv rename #66 and fix(linux): binaries fail to load with Mesa libGL (direct GL extension symbols); utvio FBO fails under llvmpipe #68 (4 tests excluded on Linux CI, utvio marked xfail).
• Windows format gaps from fix(windows): HEIC/HEIF advertised as writable but fail; FITS doesn't read back #67 (currently xfail)

Relabeled from bug to enhancement.