feat: add power gauge/delta/minmax to ac-power and extract shared utils

ac-power enhancements:
- Add visual block gauge [████░░░░] colored by power level
- Show delta indicator (+50W) colored by trend (green=dropping, red=rising)
- Track min/max values with persistence to ~/.cache/mpv/
- Multi-process safe file writes (merges values on save)
- New script-message: ac-power-reset-minmax

Shared library (scripts/lib/utils.lua):
- Extract common helpers: trim, clamp, rgb_to_bgr, colorize
- Update all scripts to use shared module via package.path

scripts/kb-blackout.lua

@@ -32,6 +32,9 @@ local msg = require("mp.msg")
 local utils = require("mp.utils")
 local jit = rawget(_G, "jit") or {}
 
+package.path = mp.command_native({ "expand-path", "~~/scripts/lib/?.lua;" }) .. package.path
+local lib = require("utils")
+
 local function coerce_boolean(value, default)
   if type(value) == "boolean" then
     return value
@@ -136,10 +139,6 @@ if unsupported_platform() then
   return
 end
 
-local function trim(str)
-  return str and str:match("^%s*(.-)%s*$") or ""
-end
-
 local function build_brightnessctl_args(extra)
   local args = { config.brightnessctl_path }
   if config.led_path ~= nil and config.led_path ~= "" then
@@ -193,7 +192,7 @@ local function run_brightnessctl(extra, action)
   end
 
   if result.status ~= 0 then
-    local stderr_text = trim(result.stderr or "")
+    local stderr_text = lib.trim(result.stderr or "")
     msg.error(("brightnessctl %s failed (exit %d): %s"):format(action, result.status, stderr_text))
     log_permission_hint(stderr_text)
     state.control_blocked = true
@@ -233,7 +232,7 @@ local function read_current_brightness()
     return nil, err
   end
 
-  local cleaned = trim(stdout)
+  local cleaned = lib.trim(stdout)
   if cleaned == "" then
     msg.error("brightnessctl returned empty output")
     return nil, "empty output"