plunger: fix multi-shell export, expand strip to 8x

- Wrap plunger_strip in union() for clean single-shell STL export
- Add SPRUE_OVL=0.2mm so bridges properly intersect shaft bodies
- Expand strip from 4 to 8 plungers (3 bridges → 7)

plunger.scad

@@ -25,18 +25,21 @@ SPRUE_GAP  = 2.0;                       // gap between adjacent flanges
 SPRUE_PITCH = FLANGE_D + SPRUE_GAP;     // centre-to-centre
 SPRUE_T    = 0.5;                        // bridge thickness — snaps cleanly
 SPRUE_Z    = FLANGE_T + 0.4;            // just above the flange shoulder
+SPRUE_OVL  = 0.2;                        // overlap into each shaft so shells merge
 
 module plunger_strip() {
-  for (i = [0:3])
-    translate([i * SPRUE_PITCH, 0, FLANGE_T])
-      rotate([90, 0, 0])
-        plunger();
+  union() {
+    for (i = [0:7])
+      translate([i * SPRUE_PITCH, 0, FLANGE_T])
+        rotate([90, 0, 0])
+          plunger();
 
-  for (i = [0:2]) {
-    x0 = i * SPRUE_PITCH + PLUNGER_D / 2;
-    x1 = (i + 1) * SPRUE_PITCH - PLUNGER_D / 2;
-    translate([x0, -PLUNGER_D / 2, SPRUE_Z])
-      cube([x1 - x0, PLUNGER_D, SPRUE_T]);
+    for (i = [0:6]) {
+      x0 = i * SPRUE_PITCH + PLUNGER_D / 2 - SPRUE_OVL;
+      x1 = (i + 1) * SPRUE_PITCH - PLUNGER_D / 2 + SPRUE_OVL;
+      translate([x0, -PLUNGER_D / 2, SPRUE_Z])
+        cube([x1 - x0, PLUNGER_D, SPRUE_T]);
+    }
   }
 }