fff: improve and split the assembly into a separate file

this makes it a lot easier to work on a part and look at it in the
assembled view by simply switching tabs. the separate file links to
the original provcase.FCStd. so, need both files to work on the assembly.

it is now possibly to simply select "exploded" or "assembled" option
from a "view" property of the top level assembly std part: no need to
manually modify the asmsheet. the property config is in the same
spreadsheet.

display standoff length now correctly offsets the LCD, the board and top
heatsink while leaving the rest in place. so, it is very easy to change
the overall height.

this change also applies 4.3inch LCD updates from 0c42295 with correct
rpi board mount screw holes, since the LCD master sketch in the
assembly is a copy.

README.md

@@ -25,13 +25,17 @@ all object constraints reference an embedded spreadsheet `params` to avoid
 prefer constraining against datum objects instead of faces and other elements produced
 directly from sketches like pads.
 
-finally, the file contains an "assembly" group. this is simply a collection of parts
-composed together to help visualize how and whether all of them fit well together.
-this assembly group calculates all placement offsets from `asmsheet` spreadsheet.
-here's how an assembly looks like:
+finally, the [fff/provcase-assembly.FCStd](fff/provcase-assembly.FCStd) file contains
+an "assembly" group. this is simply a collection of parts composed together to help
+visualize how and whether all of them fit well together. it links to the provcase.FCStd,
+so you'll need both files. this assembly group calculates all placement offsets from
+`asmsheet` spreadsheet. here's how the assembly looks like:
 
 ![provisional fff case assembly](fff/provcase-assembly.png)
 
+the "assembly" std part has a custom property "view" with two options: exploded and
+assembled. the property is configured from the same asmsheet.
+
 ### 3D printing the parts
 
 > TODO: insert here acceptable 3D printer specs and an assembly guide