fix(demo-01): repair scratch-static build (alpine 3.21 for clang19 + add static-musl preset)

Author: Scaffold Bot

examples/demo-01-image-strategy/CMakePresets.json

@@ -40,6 +40,17 @@
         "CMAKE_CXX_FLAGS_RELEASE": "-O3 -DNDEBUG -fno-plt"
       }
     },
+    {
+      "name": "static-musl",
+      "inherits": "_base",
+      "displayName": "Static musl build with clang + lld, thin LTO (scratch runtime)",
+      "cacheVariables": {
+        "CMAKE_C_COMPILER": "clang",
+        "CMAKE_CXX_COMPILER": "clang++",
+        "CMAKE_EXE_LINKER_FLAGS": "-static -fuse-ld=lld -flto=thin",
+        "CMAKE_CXX_FLAGS_RELEASE": "-O3 -DNDEBUG -flto=thin"
+      }
+    },
     {
       "name": "pgo-generate",
       "inherits": "_base",
@@ -66,6 +77,7 @@
     { "name": "release",                   "configurePreset": "release" },
     { "name": "release-static",            "configurePreset": "release-static" },
     { "name": "release-static-libstdcxx",  "configurePreset": "release-static-libstdcxx" },
+    { "name": "static-musl",               "configurePreset": "static-musl" },
     { "name": "pgo-generate",              "configurePreset": "pgo-generate" },
     { "name": "pgo-use",                   "configurePreset": "pgo-use" }
   ]

examples/demo-01-image-strategy/Containerfile.scratch-static

@@ -20,7 +20,9 @@
 # Build:
 #   podman build -f Containerfile.scratch-static -t demo-01-scratch-static .
 
-FROM docker.io/alpine:3.20 AS build
+# alpine 3.21 is the first release carrying clang19/lld19 (3.20 shipped
+# clang17); the toolchain label below pins the intent to clang 19.
+FROM docker.io/alpine:3.21 AS build
 
 # `libstdc++-static` is the actual archive (libstdc++.a) we link with -static.
 # It's separate from g++ on Alpine and isn't pulled by build-base. If the