chore: open-source release-prep (udev flag, SPDX headers, boolean style)

Three release cleanups, all behavior-preserving for my machines:

- Gated the Logitech BRIO udev rule behind INSTALL_DEVICE_UDEV_RULES (default yes, opt-out), so the device-specific rule is off for anyone without that hardware. Added the config read, validation, and a conf.example entry.
- Added a GPL-3.0-or-later SPDX-License-Identifier header after the shebang of all 24 shell scripts in the repo.
- Standardized boolean conditionals on the explicit [ "$var" = "true" ] form, replacing the bare `if $var` idiom. The STEPS function-dispatch is left alone, since it runs a function name rather than testing a boolean.

2 files changed, 2 insertions, 0 deletions

diff --git a/assets/color-themes/generate-palette.sh b/assets/color-themes/generate-palette.sh
index 456d1a4..5a11264 100755
--- a/assets/color-themes/generate-palette.sh
+++ b/assets/color-themes/generate-palette.sh
@@ -1,4 +1,5 @@
 #!/bin/sh
+# SPDX-License-Identifier: GPL-3.0-or-later
 # Generate dupre-palette.png from color definitions using ImageMagick.
 # Output: assets/color-themes/dupre/dupre-palette.png
 
diff --git a/assets/easyeffects-eq-presets.sh b/assets/easyeffects-eq-presets.sh
index 40e9cd9..9a2ecef 100755
--- a/assets/easyeffects-eq-presets.sh
+++ b/assets/easyeffects-eq-presets.sh
@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-3.0-or-later
 # Install EasyEffects parametric EQ presets (Harman target)
 #
 # Presets: