Feature: Bootstrap script for new installs

bootstrap.sh installs everything listed in system/packages.txt (pacman,
AUR via yay, flatpak, official install scripts), auto-detects CPU
microcode and GPU drivers, enables services, links dotfiles with stow
and runs system/install.sh. Optional --svt-av1 installs
svt-av1-essential-git and rebuilds ffmpeg against it via
system/rebuild-ffmpeg.sh.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

system/rebuild-ffmpeg.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# Rebuilds Arch's ffmpeg package from source so it links against the installed
+# svt-av1-essential-git instead of the repo svt-av1 it was built with.
+#
+# Re-run this after a `pacman -Syu` that updates ffmpeg, otherwise the repo
+# build replaces this one.
+set -euo pipefail
+
+if ! pacman -Qq svt-av1-essential-git >/dev/null 2>&1; then
+    echo "svt-av1-essential-git is not installed; nothing to rebuild against." >&2
+    exit 1
+fi
+
+build="$(mktemp -d)"
+trap 'rm -rf "$build"' EXIT
+
+git clone --depth 1 https://gitlab.archlinux.org/archlinux/packaging/packages/ffmpeg.git "$build/ffmpeg"
+cd "$build/ffmpeg"
+
+# The PKGBUILD verifies a signed ffmpeg tag; makepkg won't fetch the keys itself
+mapfile -t keys < <(source ./PKGBUILD && printf '%s\n' "${validpgpkeys[@]}")
+if (( ${#keys[@]} )); then
+    gpg --keyserver hkps://keyserver.ubuntu.com --recv-keys "${keys[@]}"
+fi
+
+# No --needed: the version matches what's installed, we want it replaced anyway
+makepkg --syncdeps --install --cleanbuild --noconfirm