Add the cross-build sysroot recipe, verified on the device

packaging/make-sysroot.sh populates a minimal Alpine aarch64 rootfs with
apk-tools-static, so a cross build needs no root, no qemu and no phone. Adapted
from imsd's, plus glib-dev for the bus layer that is coming.

Building against a real Alpine sysroot means the binary links dynamically
against the phone's own musl and libc++ -- libc++, libc++abi, libunwind and
libgcc_s are all already on pmOS. The harness this daemon replaces had to be
built -static, but only because it was built with the host's glibc toolchain;
that constraint was never about the target.

The pipeline is proven end to end rather than assumed: the cross-built binary
runs on the phone, and all five core suites pass there too -- on the real
hardware, not just under qemu-aarch64. That matters for modules that are almost
entirely little-endian field packing and offset arithmetic.

README.md

@@ -56,11 +56,23 @@ crafter-build test         # the unit suites
 Cross-compiling for the phone:
 
 ```sh
+packaging/make-sysroot.sh                        # once; no root, no qemu, no device
 crafter-build -- --target=aarch64-alpine-linux-musl \
-  --sysroot=<alpine-aarch64-sysroot> --march=armv8-a --mtune=generic
-crafter-build test --target=aarch64-alpine-linux-musl
+  --sysroot=~/.cache/fingerprintd/sysroot-aarch64-alpine \
+  --march=armv8-a --mtune=generic
+crafter-build test --target=aarch64-alpine-linux-musl --sysroot=... \
+  --march=armv8-a --mtune=generic                # runs the suites under qemu-aarch64
 ```
 
+The result links dynamically against the phone's own musl and libc++
+(`libc++`, `libc++abi`, `libunwind`, `libgcc_s`, all already present on pmOS).
+The research harness this replaces had to be built `-static`, but only because
+it was built with the host's glibc toolchain — that constraint does not apply
+to a real Alpine sysroot.
+
+Verified on the device: all five suites pass cross-built and run **on the phone
+itself**, not only under emulation.
+
 ## Status
 
 **The core is complete; the daemon does not run yet.** Everything was ported

packaging/make-sysroot.sh

@@ -0,0 +1,53 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-3.0-only
+# SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+# make-sysroot.sh — build an Alpine aarch64 sysroot for cross-compiling
+# fingerprintd.
+#
+# Downloads apk-tools-static + the Alpine signing keys from the CDN and
+# populates a minimal rootfs (musl, libc++, glib and their headers) that clang
+# can use via --sysroot. No root, no qemu, no device needed:
+#
+#   packaging/make-sysroot.sh [dir]     # default: ~/.cache/fingerprintd/sysroot-aarch64-alpine
+#   crafter-build -- --target=aarch64-alpine-linux-musl --sysroot=<dir> \
+#       --march=armv8-a --mtune=generic
+#
+# Cross-building against a real Alpine sysroot is why the binary links
+# DYNAMICALLY against the phone's own musl and libc++. The research harness
+# this daemon replaces had to be built -static, but only because it was built
+# with the host's glibc toolchain; that constraint does not apply here.
+set -eu
+
+SYSROOT="${1:-$HOME/.cache/fingerprintd/sysroot-aarch64-alpine}"
+MIRROR="${MIRROR:-https://dl-cdn.alpinelinux.org/alpine/edge}"
+ARCH=aarch64
+# gcc is in the list for its crtbeginS.o/libgcc: clang's driver for
+# *-alpine-linux-musl links against the GCC runtime it detects under
+# <sysroot>/usr/lib/gcc/<triple>/.
+# glib-dev is for the GDBus bus layer (net.reactivated.Fprint).
+PKGS="musl-dev libc++ libc++-dev glib-dev compiler-rt llvm-libunwind-dev gcc"
+
+work=$(mktemp -d)
+trap 'rm -rf "$work"' EXIT
+
+fetch_pkg() {  # $1 = repo (main/community), $2 = package name -> path on stdout
+    file=$(curl -fsSL "$MIRROR/$1/x86_64/" |
+        grep -o "href=\"$2-[0-9][^\"]*\.apk\"" | head -n1 | cut -d'"' -f2)
+    [ -n "$file" ] || { echo "cannot find $2 in $MIRROR/$1" >&2; exit 1; }
+    curl -fsSL -o "$work/$file" "$MIRROR/$1/x86_64/$file"
+    echo "$work/$file"
+}
+
+echo ">> fetching apk-tools-static + alpine-keys"
+tar -xzf "$(fetch_pkg main apk-tools-static)" -C "$work" sbin/apk.static 2>/dev/null
+tar -xzf "$(fetch_pkg main alpine-keys)" -C "$work" usr/share/apk/keys 2>/dev/null
+
+echo ">> populating $SYSROOT ($ARCH: $PKGS)"
+mkdir -p "$SYSROOT"
+"$work/sbin/apk.static" \
+    --arch "$ARCH" --root "$SYSROOT" --initdb --no-scripts --no-cache \
+    --usermode --keys-dir "$work/usr/share/apk/keys/$ARCH" \
+    -X "$MIRROR/main" -X "$MIRROR/community" \
+    add $PKGS
+
+echo ">> done: $SYSROOT"