feat(desktop): version source-of-truth + RPM + AppImage packaging

Phase 1+2 of multi-platform distribution plan:
- gradle/libs.versions.toml: add `app = "1.08.0"` as single source of truth
- Root build.gradle: allprojects { version = libs.versions.app.get() }
- amethyst/build.gradle: versionName from catalog (versionCode stays local)
- desktopApp/build.gradle.kts: drop hardcoded "1.0.0"; inherit project.version;
  add TargetFormat.Rpm; add linux DSL (menuGroup, appCategory, debMaintainer,
  rpmLicenseType, rpmPackageVersion with dashes stripped)
- desktopApp/build.gradle.kts: new createReleaseAppImage task wrapping
  createReleaseDistributable with linuxdeploy (TargetFormat.AppImage is
  broken in Compose 1.10.x — CMP-7101)
- packaging/appimage/: AppRun launcher (sets LD_LIBRARY_PATH for bundled VLC),
  amethyst.desktop XDG entry, 512x512 icon extracted from icon.icns
- scripts/asset-name.sh: single source for release asset naming contract

scripts/asset-name.sh

@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+# Single source of truth for Amethyst Desktop release asset naming.
+#
+# Contract: amethyst-desktop-<version>-<family>-<arch>.<ext>
+#   <version> = tag stripped of leading 'v' (e.g. "1.08.0")
+#   <family>  = macos | windows | linux
+#   <arch>    = x64 | arm64
+#   <ext>     = dmg | msi | zip | deb | rpm | AppImage | tar.gz
+#
+# Consumed by: .github/workflows/create-release.yml, .github/workflows/bump-*.yml,
+#              BUILDING.md, local release runbooks.
+#
+# Usage:
+#   source scripts/asset-name.sh
+#   collect_assets <family> <arch> <version> <dest_dir>
+#
+# Expected examples:
+#   amethyst-desktop-1.08.0-macos-x64.dmg
+#   amethyst-desktop-1.08.0-macos-arm64.dmg
+#   amethyst-desktop-1.08.0-windows-x64.msi
+#   amethyst-desktop-1.08.0-windows-x64.zip
+#   amethyst-desktop-1.08.0-linux-x64.deb
+#   amethyst-desktop-1.08.0-linux-x64.rpm
+#   amethyst-desktop-1.08.0-linux-x64.AppImage
+#   amethyst-desktop-1.08.0-linux-x64.tar.gz
+
+set -euo pipefail
+
+# Print the canonical asset filename for a given family/arch/version/extension.
+# Usage: asset_name <family> <arch> <version> <ext>
+asset_name() {
+    local family="$1" arch="$2" version="$3" ext="$4"
+    printf 'amethyst-desktop-%s-%s-%s.%s' "$version" "$family" "$arch" "$ext"
+}
+
+# Copy + rename build outputs into <dest_dir> using the canonical naming scheme.
+# Usage: collect_assets <family> <arch> <version> <dest_dir>
+# Expects build outputs under desktopApp/build/... (Compose binaries + custom tasks + portable archives).
+collect_assets() {
+    local family="$1" arch="$2" version="$3" dest="$4"
+    mkdir -p "$dest"
+    shopt -s nullglob
+
+    # Compose Desktop jpackage outputs (main-release/<format>/*.ext)
+    local src ext base dst
+    for src in \
+        desktopApp/build/compose/binaries/main-release/dmg/*.dmg \
+        desktopApp/build/compose/binaries/main-release/msi/*.msi \
+        desktopApp/build/compose/binaries/main-release/deb/*.deb \
+        desktopApp/build/compose/binaries/main-release/rpm/*.rpm \
+        desktopApp/build/appimage/*.AppImage \
+        desktopApp/build/portable/*.tar.gz \
+        desktopApp/build/portable/*.zip \
+    ; do
+        [ -f "$src" ] || continue
+        base="$(basename "$src")"
+        case "$base" in
+            *.tar.gz) ext="tar.gz" ;;
+            *)        ext="${base##*.}" ;;
+        esac
+        dst="$dest/$(asset_name "$family" "$arch" "$version" "$ext")"
+        cp "$src" "$dst"
+        echo "Collected: $dst"
+    done
+    shopt -u nullglob
+}