Close power switch exterior face; add PWR_COVER_T param

Switch is not user-accessible so the slot is now an interior pocket
with a 0.6 mm exterior skin. Adds TODO to calibrate PWR dimensions
from STL before next print.

cpu_case.scad

@@ -23,10 +23,12 @@ COMP_SIDE_Z = 3.82;   // tallest component on +Y wall (buttons, USB-C)
 /* [Cutout Positions — from STL geometry, PCB-origin-relative] */
 
 // ---- +Y wall: Sliding power switch ----
+// TODO: measure actual switch body dimensions from STL (as done for USB-C) before next print
 PWR_CX      =  10.3;   // PCB-relative case_x centre
 PWR_W       =   5.0;   // slot width
 PWR_H       =   2.5;   // slot height
 PWR_CZ      =   3.8;   // height above PCB bottom
+PWR_COVER_T =   0.6;   // exterior skin over switch pocket (switch not user-accessible)
 
 // ---- +Y wall: USB-C port ----
 USBC_CX         =  65.65;
@@ -159,11 +161,11 @@ module base() {
 
             // ---- +Y wall cutouts ----
 
-            // Sliding power switch slot
+            // Sliding power switch pocket — closed exterior face, interior clearance only
             translate([PCB_OX + PWR_CX - PWR_W/2,
                        OUT_W - WALL - 0.1,
                        PCB_Z + PWR_CZ - PWR_H/2])
-                cube([PWR_W, WALL + 0.2, PWR_H]);
+                cube([PWR_W, WALL - PWR_COVER_T + 0.1, PWR_H]);
 
             // USB-C slot
             translate([PCB_OX + USBC_CX - (USBC_W + 2*USBC_CBL_CLEAR)/2,