SIGBUS crash on click (Intel HD 4000, Mesa 26.1.4) — QSGRhiLayer::grab() in gallium driver

## Description

quickshell/DMS crashes with SIGBUS (signal 7) on user click interaction. The crash occurs in Qt Quick's `QSGRhiLayer::grab()` → `ensurePipelineState()` path, inside Mesa's gallium i915 driver.

## Environment

- **GPU**: Intel HD Graphics 4000 (Ivy Bridge, PCI ID `8086:0166`)
- **Mesa**: `26.1.4-1` (Arch Linux)
- **Qt**: `6.11.1`
- **quickshell**: git build
- **dms-shell**: `1.4.6-3`
- **Compositor**: niri 25.x
- **Render loop**: threaded (via `QSG_RENDER_LOOP=threaded` pragma in shell.qml)
- **RHI backend**: OpenGL (`QSG_RHI_BACKEND=opengl`)

## Crash Backtrace

```
#0 libc (syscall)
#1 raise() (called by quickshell crash handler)
#2 quickshell::crash::signalHandler()
#3 signal delivery trampoline
#4 libgallium-26.1.4-arch1.1.so
#5 libgallium
#6 libgallium
#7 libgallium
#8 Qt6Gui (QRhiGles2::sanityCheckBoundFbo?)
#9 Qt6Gui
#10 QSGBatchRenderer::Renderer::ensurePipelineState()
#11 Qt6Quick
#12 QSGBatchRenderer::Renderer::prepareRenderPass()
#13 QSGBatchRenderer::Renderer::render()
#14 Qt6Quick
#15 QSGRhiLayer::grab()
#16 QSGRhiLayer::updateTexture()
#17 QSGBasicInternalImageNode::preprocess()
```

All 18+ coredumps share this identical backtrace. Crash always triggered on click (opening popups/overviews/control-center), never on idle.

## Workaround

Setting the environment variable `DMS_DISABLE_LAYER=true` prevents the crash by disabling Qt Quick `layer.enabled: true` in DMS widgets that check for this env var. However, many QML files (`ChangelogContent.qml`, `ClipboardThumbnail.qml`, `PowerMenuModal.qml`, `DockItem.qml`, `DankBarItem.qml`, `DesktopWidgetLayer.qml`, etc.) still use `layer.enabled: true` unconditionally, so they may still trigger the crash.

See also: `DankOSD.qml`, `DankPopout.qml`, `DankSlideout.qml` — these already check `Quickshell.env("DMS_DISABLE_LAYER")`.

## Possible Solutions

1. **Expand `DMS_DISABLE_LAYER` coverage**: Add the env var check to all remaining QML files that use `layer.enabled: true`, so users can fully opt out of layer-based rendering on affected GPUs.
2. **Mesa regression**: The crash is inside `libgallium-26.1.4` — downgrading Mesa may help.
3. **Vulkan vs OpenGL**: Both RHI backends crash on this GPU. forcing `LIBGL_ALWAYS_SOFTWARE=true` stops the crash but kills performance.

Let me know if you need more info (coredump files, full env, etc.). I can provide them.

Review the layer.enabled usages in ChangelogContent.qml, ClipboardThumbnail.qml, PowerMenuModal.qml, DockItem.qml, DankBarItem.qml, and DesktopWidgetLayer.qml, comparing them with DankOSD.qml, DankPopout.qml, and DankSlideout.qml. Verify that DMS_DISABLE_LAYER covers the remaining affected widgets, then test click-triggered popups and overviews with the variable enabled on the reported setup.

Written by the indexing model from the issue text.