docs(case): document the plan section and embed its render

New `show="plan"` lifts the deck/screen half off the cavity half so the
board layout reads at a glance — standoffs, corner posts, ports, and the
internal FPC clearance. Tunable via `plan_z` and `explode`.

hardware/case/README.md

@@ -41,7 +41,8 @@ openscad -o renders/assembled.png --imgsize=1100,825 --colorscheme=Tomorrow \
 openscad -o body.stl -D 'show="body"' typoena-case.scad
 ```
 
-`show` accepts `assembled` · `body` · `bracket` · `baseplate` · `print_plate`.
+`show` accepts `assembled` · `body` · `bracket` · `baseplate` · `print_plate` ·
+`section` (vertical cut) · `plan` (horizontal cut into the cavity).
 
 ## Dimensions
 
@@ -118,6 +119,13 @@ from below — far easier than fishing screws inside a shell.
 
 ![The baseplate — four standoffs for the ESP32 (centre) and two for the DESPI-C579 breakout (left, under the FPC exit); mount at the bench, then close from below](renders/baseplate.png)
 
+Exploded horizontal section (`show="plan"`) — the deck/screen half lifts off
+the cavity half so you see both at once: ESP32 standoffs centre, DESPI-C579
+standoffs left under the FPC exit, the two back corner posts, and the three
+port openings in the back wall.
+
+![Exploded plan section: deck/screen half lifted above the cavity half — standoffs, corner posts, back-wall ports](renders/plan.png)
+
 ### Assembly order
 
 1. Lay glass into the deck recess (from inside), add the foam gasket, screw the

hardware/case/typoena-case.scad

@@ -18,10 +18,11 @@
 //    "bracket"     – the screen retaining frame (print flat)
 //    "baseplate"   – the chassis / bottom cover (print flat)
 //    "print_plate" – all printed parts laid out side by side
-//    "section"     – midline cross-section: how the screen is trapped
+//    "section"     – vertical cross-section: how the screen is trapped
+//    "plan"        – exploded horizontal section: deck lifted off the cavity
 // ============================================================================
 
-show = "section";
+show = "plan";
 $fn = 48;
 
 // ---- body envelope --------------------------------------------------------
@@ -280,6 +281,14 @@ module placed_foam() {
     on_deck() translate([screen_off, screen_cy+screen_off, -lip_t-G_t-foam_t])
         color(C_foam) foam();
 }
+// full coloured assembly, reused by the exploded plan section
+module plan_assembly() {
+    color(C_body)   case_body();
+    ghost_screen();
+    placed_foam();
+    placed_bracket();
+    translate([0,0,-0.01]) color(C_plate) baseplate();
+}
 
 if (show == "assembled") {
     color(C_body)   case_body();
@@ -297,7 +306,7 @@ if (show == "assembled") {
     translate([W+30, 0, 0])           color(C_plate)   baseplate();
     translate([W+30, D+30, foot_h])   color(C_bracket) bracket();
 } else if (show == "section") {
-    // slice away the +X half: the cut face shows the screen clamp, and the
+    // VERTICAL slice (remove +X half): cut face shows the screen clamp, and the
     // retained LEFT half exposes the internal FPC clearance behind the bezel
     difference() {
         union() {
@@ -309,4 +318,18 @@ if (show == "assembled") {
         }
         translate([W/2, -30, -70]) cube([W, D+60, 220]);
     }
-}
+} else if (show == "plan") {
+    // HORIZONTAL slice at plan_z, shown EXPLODED: the bottom half (cavity —
+    // baseplate standoffs, corner posts, back-wall ports) stays put; the top
+    // half (deck, screen, bracket) lifts up so you see both sides of the cut.
+    plan_z  = 30;
+    explode = 62;
+    intersection() {                                   // bottom: the cavity
+        plan_assembly();
+        translate([-60, -60, plan_z-200]) cube([W+120, D+120, 200]);
+    }
+    translate([0, 0, explode]) intersection() {        // top: the deck, lifted
+        plan_assembly();
+        translate([-60, -60, plan_z]) cube([W+120, D+120, 200]);
+    }
+}