Waterfall QImage uploaded as RGBA8 texture, rendered as textured quad
with ring buffer scrolling via fragment shader fract(uv.y + offset).
All overlays (grid, FFT, band plan, slices, TNF, spots, scales) painted
into an overlay QImage via QPainter, composited as alpha-blended texture.

Key fix: fract() must be in the fragment shader, not vertex shader.
Per-vertex fract() linearizes between two identical values when the UV
range spans exactly 1.0 (fract(0+offset) == fract(1+offset)), producing
constant UV across the quad. This was likely the same bug that killed
the previous GPU attempt.

Shaders compiled at build time via qt_add_shaders() — no binary .qsb
files in the repo, shader errors are build errors not silent runtime
failures.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Replace full 3.5MB QImage upload per frame with per-row incremental
uploads (~6KB each). Track last uploaded row, walk the ring buffer
delta, upload only changed rows with setDestinationTopLeft().

CPU reduction: 97% (QPainter) → 52% (full upload) → 40% (incremental).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Split overlay into static (grid, band plan, scales, slices, TNF, spots)
and dynamic (FFT spectrum line) layers. Static overlay only repainted
when markOverlayDirty() is called from state-change methods. Per-frame
work: memcpy cached static, paint FFT on top, upload composite.

Replaced all update() calls with markOverlayDirty() except the two
data paths (updateSpectrum, updateWaterfallRow) which only need a
render, not a static overlay repaint.

CPU progression: 97% (QPainter) → 52% (full upload) → 40% (incremental
waterfall) → 35% (static overlay caching).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

markOverlayDirty() was declared inside #ifdef AETHER_GPU_SPECTRUM but
called from non-guarded code. The method handles both paths internally
so it must be visible regardless of the GPU flag.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

AETHER_GPU_SPECTRUM now defaults to ON for shipped binaries. Added
qt6-base-private-dev, qt6-shader-baker, and qt6-shadertools-dev to
the CI Docker image.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

GPU spectrum defaults to ON but falls back to OFF if Qt6::ShaderTools
or Qt6::GuiPrivate aren't found, instead of failing the build. Allows
CI to build successfully while the Docker image is being updated.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Qt6::ShaderTools and Qt6::GuiPrivate must be present. Build fails
loudly if missing, forcing the CI image to be updated rather than
silently shipping without GPU acceleration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

All release builds (AppImage, Windows, macOS DMG, macOS PKG) were
missing qtwebsockets (TCI server), qtshadertools (GPU spectrum).
ARM AppImage also missing serialport, hidapi, fftw3, portaudio,
qt6-base-private-dev.

This is why TCI wasn't in shipped binaries — the aqtinstall modules
list only had qtmultimedia and qtserialport.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

All packages listed for Arch, Ubuntu, Fedora, macOS with collapsible
table explaining what each dependency enables. Includes GPU spectrum
(qt6-shadertools), TCI (qt6-websockets), StreamDeck (hidapi), NR2
(fftw3) and all other auto-detected features.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

QRhiWidget requires Qt 6.7+. CI Docker image has Qt 6.4 (Ubuntu 24.04)
which doesn't have QRhiWidget. GPU spectrum now auto-disables on older
Qt instead of failing the build. Release workflows use Qt 6.7 via
aqtinstall and get GPU enabled.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>