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/lib/utils.lua

@@ -0,0 +1,44 @@
+--[[
+  Shared utility functions for mpv scripts.
+]]
+
+local M = {}
+
+-- String utilities
+function M.trim(str)
+  return str and str:match("^%s*(.-)%s*$") or ""
+end
+
+-- Math utilities
+function M.clamp(value, min_v, max_v)
+  if value < min_v then
+    return min_v
+  end
+  if value > max_v then
+    return max_v
+  end
+  return value
+end
+
+-- ASS color utilities
+function M.rgb_to_bgr(rgb)
+  return rgb:sub(5, 6) .. rgb:sub(3, 4) .. rgb:sub(1, 2)
+end
+
+function M.colorize(text, color_rgb, use_colors)
+  if use_colors == false then
+    return text
+  end
+  return "{\\1c&H" .. M.rgb_to_bgr(color_rgb) .. "&}" .. text .. "{\\1c&HFFFFFF&}"
+end
+
+-- Debug logging factory
+function M.make_dbg(prefix, log_fn)
+  return function(text, config_enabled)
+    if config_enabled ~= false then
+      log_fn(prefix .. ": " .. text)
+    end
+  end
+end
+
+return M