linux-ampere-fourier: bootstrap PKGBUILD (CoolPi GenBook RK3588)

Sibling to kernel-agent PR #8 which lands the scope-tagged board
patches + fleet/ampere.yaml manifest. This PR carries the
makepkg-side recipe.

## Baseline

marfrit/linux-rk3588-marfrit @ f8f3ad9 (mainline v7.0-rc3 + 18 commits):
  10 by Markus Fritsche  (board DTS + Kconfig + suspend/wakeup + bt)
   4 by Shawn Lin        (phy: rockchip-snps-pcie3 stability)
   2 by Cristian Ciocaltea (clk + dw-dp bridge — Collabora)
   1 by Sebastian Reichel (Rock 5 ITX hdmirx — unused by ampere)
   1 by Pedro Alves      (CLK_SET_RATE_PARENT VOP2 fix)

The 6 board-relevant patches (pwm15 pinctrl + RK806 power-off +
genbook pwm-fan/speaker/USB-C/lid) are scope-tagged in
kernel-agent. The 12 others are 'cherry-picks-in-baseline'
currently — splitting them into scope-tagged patches is a
follow-up.

## Source distribution

The kernel tree isn't pushable to Gitea (boltzmann's working clone
is shallow). Tarball (260MB) doesn't belong in marfrit-packages
either. Pragmatic shortcut: ship prebuild.sh that produces the
tarball locally from $LINUX_RK3588_MARFRIT_TREE (default
~/src/linux-rockchip) just before makepkg runs. PKGBUILD source
array references the tarball by name; makepkg picks it up from
the PKGBUILD dir.

Build flow:
  cd arch/linux-ampere-fourier
  ./prebuild.sh           # produces 260MB tarball; idempotent w/ sha check
  makepkg -s --noconfirm

Future iteration: host the tarball on Gitea release assets or
packages.reauktion.de/sources/ and switch source to URL.

## What the PR ships

  PKGBUILD                          — the recipe (linux-rk3588-marfrit @ f8f3ad9)
  config                            — snapshot of ampere's /proc/config.gz
  linux-ampere-fourier.preset       — mkinitcpio preset, /boot/firmware/ paths
  extlinux-add.hook                 — alpm hook, fires on /boot/firmware/Image* changes
  extlinux-add.sh                   — idempotent managed-label injector for
                                     /boot/firmware/extlinux/extlinux.conf
                                     (vfat on mmcblk0p1, ampere-specific path)
  prebuild.sh                       — source-staging helper (see above)
  README.md                         — build + install runbook

## ampere-specific boot path differences vs fresnel

ampere boots from /boot/firmware/ (vfat partition), not /boot/ (root
partition) like fresnel. The PKGBUILD installs Image + initramfs + DTB
under /boot/firmware/ directly. The extlinux-add hook fires on
/boot/firmware/<X> path changes and edits
/boot/firmware/extlinux/extlinux.conf. The mkinitcpio preset writes
initramfs to /boot/firmware/.

Only one PRESET ('default') — no fallback image — because
/boot/firmware is ~1G total and two ~20MB initramfs files plus the
DTB and a couple of kernel-image backups would squeeze it tight.

## Out of scope this PR

- Ask 2 (VP9 enablement) and Ask 3 (AV1 dec) from kernel-agent issue #6
- Build + publish + install (sequenced after this PR + kernel-agent PR #8 merge)
- Splitting the 12 non-board commits into scope-tagged kernel-agent patches

arch/linux-ampere-fourier/prebuild.sh

@@ -0,0 +1,65 @@
+#!/bin/bash
+# prebuild — stage the kernel source tarball this PKGBUILD expects.
+#
+# linux-ampere-fourier's source is a snapshot of marfrit/linux-rk3588-marfrit
+# @ f8f3ad9 (260MB), too big to commit to marfrit-packages and currently
+# unpushable to Gitea (boltzmann's working clone is shallow; gitea push
+# refuses shallow updates). Hosting the tarball outside Gitea would need
+# infrastructure setup that's not in scope for the first iteration.
+#
+# So: produce the tarball locally from the kernel working tree just
+# before makepkg runs. Idempotent — if an existing tarball matches the
+# expected sha256 we skip the archive step.
+#
+# Run from this directory:  cd arch/linux-ampere-fourier && ./prebuild.sh
+# Override the kernel-tree location:  LINUX_RK3588_MARFRIT_TREE=/path ./prebuild.sh
+#
+# Default tree location matches the boltzmann/ampere convention from
+# kernel-agent issue #6: $HOME/src/linux-rockchip.
+set -euo pipefail
+
+TREE="${LINUX_RK3588_MARFRIT_TREE:-${HOME}/src/linux-rockchip}"
+COMMIT=f8f3ad934433bd7e1207d9b0b37e817a692b7ee9
+SHA256_EXPECTED=b4eca11e883fe6f7f306d8751c3efa3afed9cc3465c74a3320de1b7204f5f330
+
+HERE="$(cd "$(dirname "$0")" && pwd)"
+OUTPUT="${HERE}/linux-rk3588-marfrit-${COMMIT:0:7}.tar.gz"
+
+if [ -f "$OUTPUT" ]; then
+    have=$(sha256sum "$OUTPUT" | cut -d' ' -f1)
+    if [ "$have" = "$SHA256_EXPECTED" ]; then
+        echo "prebuild: $OUTPUT already exists with correct sha256"
+        exit 0
+    else
+        echo "prebuild: existing $OUTPUT sha mismatch (have=$have, want=$SHA256_EXPECTED) — regenerating" >&2
+        rm -f "$OUTPUT"
+    fi
+fi
+
+if [ ! -d "$TREE/.git" ]; then
+    echo "prebuild: kernel tree not at $TREE" >&2
+    echo "          set LINUX_RK3588_MARFRIT_TREE=/path/to/linux-rockchip and retry" >&2
+    exit 2
+fi
+
+cd "$TREE"
+
+if ! git cat-file -e "$COMMIT" 2>/dev/null; then
+    echo "prebuild: commit $COMMIT not found in $TREE" >&2
+    echo "          fetch the linux-rk3588-marfrit branch first:" >&2
+    echo "          git fetch <remote> linux-rk3588-marfrit" >&2
+    exit 3
+fi
+
+echo "prebuild: generating archive from $TREE @ $COMMIT..."
+git archive --format=tar.gz --prefix=linux-rk3588-marfrit/ "$COMMIT" -o "$OUTPUT"
+
+# git archive emits a deterministic tar stream but gzip compression may
+# vary by version. The sha256 check is informational; warn-don't-fail.
+have=$(sha256sum "$OUTPUT" | cut -d' ' -f1)
+if [ "$have" != "$SHA256_EXPECTED" ]; then
+    echo "prebuild: WARNING $OUTPUT sha=$have (canonical=$SHA256_EXPECTED)" >&2
+    echo "          probably a gzip-version difference; tar payload should be identical" >&2
+fi
+
+echo "prebuild: wrote $OUTPUT ($(du -h "$OUTPUT" | cut -f1), sha=$have)"