Kotlin/Native Linux cells, and the Linux chrome and reader checks, in the native-image gate

[b-re-w]

Closes #64

Kotlin/Native cells

• "Build (Kotlin/Native, linux-x64 and linux-arm64)" builds extended/native-desktop/hello-awtfree as an Amper module of the core fork's window project (copied into extended/window/samples/ of the checkout, scene built as its Linux module). Its content is hello-awtfree's own Content.kt and TextSelfCheck.kt, linked. A klib is read only by the compiler that wrote it or a newer one, which is why it uses the core fork's toolchain rather than a Gradle build. arm64 is cross compiled on x86-64 (Kotlin/Native has no arm64 Linux host) and linked against the arm64 desktop libraries from the ports archive.
• "One executable (Kotlin/Native, linux-x64)" and "(Kotlin/Native, linux-arm64)" run check-one-executable.sh on their own architecture, as the GraalVM cells do, then the chrome and reader check.

Linux chrome and reader check (FR-19.4, FR-19.7, NFR-8)

check-linux-chrome.sh, on every Linux cell (GraalVM x64 and arm64, Kotlin/Native x64 and arm64), on a virtual display and its own session bus with openbox and at-spi2:

• no window manager title bar under the default WindowChrome.Modern (_NET_FRAME_EXTENTS all zero);
• a caption drag moves the window by the pointer's distance, each edge and corner resizes from that side only, a double click maximises and a second restores (core fork extended/window/scripts/wm-chrome.sh);
• the window's own count: at least eight sizes, stale=0, stretched=0, and no frame presented at a size other than the window's;
• libatspi walks the tree (core fork extended/window/scripts/atspi-readback.c): entry "Name", label "Name", push button "click me", the caption's Minimize/Maximize/Close; sets the field, presses the button, and closes the window through Close.

hello-awtfree: content moved to Content.kt, the field labelled "Name", HELLO_RUN_SECONDS mode for the driven check. The scene platform patch now matches any list, since the scene declares the Linux targets (core fork JetBrains#146).

Pinned core commit

CORE_EXTENDED_REF points at the core fork's feat/linux-window-chrome head (core fork JetBrains#149, which has JetBrains#144 and JetBrains#146 merged in). It moves to the extended merge commit once those merge; this PR merges after that.

Not covered here

• The Kotlin/Native surface for the scene (X11GlWindowSurface) is in the sample until native/linux can depend on the scene module; it uses X11Window.makeCurrent() and resizePainter from the core fork.
• The macOS Kotlin/Native cell is the macOS agent's, in this same file.
• Orca speech and the feel of a drag are hand checks on a desktop.