Fix unbound BASH_SOURCE under bash -c invocation, the actual documented run command

BASH_SOURCE has zero elements when a script arrives as `bash -c "$(curl ...)"`
(no backing file at all) rather than a real path or /dev/fd process-
substitution handle. Under set -u this crashed immediately on the REPO_DIR
line, and the bottom sourcing guard's $BASH_SOURCE/$0 comparison would have
silently never run main() either way (bash -c sets $0 to "bash", which never
matches an empty BASH_SOURCE default).

Guarded the REPO_DIR line with a safe default, and replaced the sourcing
guard with the standard `return 0` idiom, which correctly distinguishes
sourced (bats tests) from directly-executed (file, bash -c, process
substitution) in every invocation style. Added a regression test that runs
the actual `bash -c "$(cat install.sh)"` command end-to-end against the real
public repo, not just sourced functions.

install.sh

@@ -1,7 +1,11 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-REPO_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# ${BASH_SOURCE[0]} is unset (not just empty) when this script arrives as a
+# `bash -c "$(curl ...)"` command string rather than a real file -- guard it
+# so `set -u` doesn't blow up. `dirname ""` -> "." (current dir), which
+# ensure_repo_files() below correctly treats as "no local checkout, fetch."
+REPO_DIR="$(cd "$(dirname "${BASH_SOURCE[0]:-}")" && pwd)"
 CONFIG_HOME="${CONFIG_HOME:-${XDG_CONFIG_HOME:-$HOME/.config}}"
 LOCAL_BIN="${LOCAL_BIN:-$HOME/.local/bin}"
 HYPREVO_RAW_BASE="${HYPREVO_RAW_BASE:-https://git.edemilov.ru/edemilov/hyprevo/raw/branch/master}"
@@ -266,6 +270,12 @@ main() {
     echo "hyprevo: done."
 }
 
-if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
+# `return` only succeeds at top level when the script is being sourced (as
+# tests/install.bats does). Direct execution -- a real file, `bash -c "..."`,
+# or `bash <(curl ...)` process substitution -- all fail `return`, so `main`
+# runs. This is more robust here than comparing $BASH_SOURCE/$0: under
+# `bash -c "$(curl ...)"` there's no backing file at all, so BASH_SOURCE is
+# empty and $0 is just "bash" -- they'd never match even with a safe default.
+if ! (return 0 2>/dev/null); then
     main "$@"
 fi

tests/install.bats

@@ -298,3 +298,39 @@ EOF
 
     rm -rf "$tmp_home" "$stub_bin"
 }
+
+@test "install.sh runs end-to-end via bash -c \"\$(cat install.sh)\" (the real distribution command)" {
+    # Regression test: BASH_SOURCE has zero elements under `bash -c "..."`
+    # (no backing file at all), which broke both the REPO_DIR line and the
+    # sourcing guard under `set -u`. Exercise the actual documented install
+    # command, not just sourced functions, so this can't silently regress.
+    local tmp_home
+    tmp_home="$(mktemp -d)"
+    local stub_bin
+    stub_bin="$(mktemp -d)"
+
+    for cmd in pacman systemctl hyprctl sudo; do
+        cat > "$stub_bin/$cmd" <<EOF
+#!/usr/bin/env bash
+echo "$cmd \$*" >> "$tmp_home/log"
+if [ "$cmd" = "sudo" ]; then exec "\$@"; fi
+exit 0
+EOF
+        chmod +x "$stub_bin/$cmd"
+    done
+
+    PATH="$stub_bin:$PATH" \
+    HOME="$tmp_home" \
+    HYPREVO_NONINTERACTIVE=1 \
+    HYPREVO_TERMINAL=Konsole \
+    HYPREVO_FILEMANAGER=Dolphin \
+    HYPREVO_MAIL=Thunderbird \
+    HYPREVO_CLOUDSYNC=Nextcloud \
+    run bash -c "$(cat "${BATS_TEST_DIRNAME}/../install.sh")"
+
+    [ "$status" -eq 0 ]
+    [[ "$output" == *"hyprevo: done."* ]]
+    [ -f "$tmp_home/.config/hypr/hyprland.conf" ]
+
+    rm -rf "$tmp_home" "$stub_bin"
+}