daedalus-v4l2-dkms: loud warning when DKMS build doesn't land

Sonnet pre-deployment review caught a BLOCKER: on a fresh higgs
(Debian 13 / Pi CM5) install without the RPi kernel headers
pre-installed, the postinst's `dkms autoinstall || true` silently
swallowed the build failure. Package appeared installed but the
.ko was absent; `modprobe daedalus_v4l2` then failed and the
entire stack was dead with no clear pointer to the cause.

Fix in both ecosystems:

debian/daedalus-v4l2-dkms/build-deb.sh:
- After `dkms autoinstall`, verify the post-condition with
  `dkms status -m daedalus_v4l2 -v VER -k $(uname -r)`.
- If the module isn't 'installed' / 'loaded' for the running
  kernel, emit a yellow-bolded ANSI warning naming the most
  likely cause (kernel headers package missing) and the exact
  recovery steps (linux-headers-rpi-2712 for RPi or
  linux-headers-$KERNELVER for Debian generic, then
  `dkms autoinstall` + `modprobe`).
- Colour only on TTY; the warning is unconditional regardless.

arch/daedalus-v4l2-dkms/:
- New daedalus-v4l2-dkms.install with post_install +
  post_upgrade hooks that run the same `dkms status` check.
- post_upgrade catches the case where a kernel-headers package
  was uninstalled / pruned between upgrades, silently
  regressing the build.
- Wired into the PKGBUILD via install="${pkgname}.install".

Both versions point at the actual repair commands rather than
just saying "build failed", so the user is one apt/pacman away
from a working stack instead of debugging dkms internals.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

arch/daedalus-v4l2-dkms/PKGBUILD

@@ -28,10 +28,13 @@ url="https://git.reauktion.de/reauktion/daedalus-v4l2"
 license=('GPL-2.0-or-later')
 depends=('dkms')
 makedepends=('git')
+install="${pkgname}.install"
 
 source=("git+https://git.reauktion.de/reauktion/daedalus-v4l2.git#commit=${_commit}"
-        "dkms.conf")
+        "dkms.conf"
+        "${pkgname}.install")
 sha256sums=('SKIP'
+            'SKIP'
             'SKIP')
 
 pkgver() {

arch/daedalus-v4l2-dkms/daedalus-v4l2-dkms.install

@@ -0,0 +1,61 @@
+# post-install / post-upgrade hook for daedalus-v4l2-dkms.
+#
+# pacman + the dkms-helpers alpm hook will already attempt
+# `dkms install` on its own.  This script's job is to emit a
+# loud, actionable warning when the module didn't actually
+# build for the running kernel — most commonly because the
+# kernel headers package isn't installed yet.
+#
+# Without this you get a silent failure: the package looks
+# installed but `modprobe daedalus_v4l2` returns ENOENT.
+
+_check_dkms_built() {
+    local name=daedalus_v4l2
+    local ver=$1
+    local kernelver=$(uname -r)
+
+    if ! command -v dkms >/dev/null 2>&1; then
+        return 1  # the hard-dep should have caught this
+    fi
+
+    local status
+    status=$(dkms status -m "$name" -v "$ver" -k "$kernelver" 2>/dev/null || true)
+    if printf '%s\n' "$status" | grep -q -E 'installed|loaded'; then
+        return 0  # all good
+    fi
+
+    cat >&2 <<EOF
+==> daedalus-v4l2-dkms: DKMS build did NOT land for kernel $kernelver.
+==>   dkms status -m $name -v $ver -k $kernelver:
+==>     $(printf '%s' "$status" | head -1)
+==>
+==> Most likely cause: kernel headers package is missing.
+==>   Arch / ALARM:     pacman -S linux-rpi-headers (or linux-rpi5-headers)
+==>   Raspberry Pi OS:  apt install linux-headers-rpi-2712
+==>
+==> After installing headers, finish the install with:
+==>   sudo dkms autoinstall $name/$ver
+==>   sudo modprobe daedalus_v4l2
+==>
+==> Until then daedalus_v4l2 will NOT be loadable and the
+==> userspace daedalus-v4l2 daemon will have nothing to talk to.
+EOF
+    return 1
+}
+
+post_install() {
+    _check_dkms_built "$1" || true
+}
+
+post_upgrade() {
+    # New version pinned by the bump may have built fine, but if
+    # a kernel-headers package was uninstalled / pruned since the
+    # last upgrade we'd silently regress.  Re-check.
+    _check_dkms_built "$1" || true
+}
+
+pre_remove() {
+    # The dkms alpm hook handles dkms remove on its own; nothing
+    # we need to add here.
+    :
+}