delete fff version and rename diyplus into "nerd"

this is to align with the naming scheme we came up on the website at
https://nakamochi.io/#editions

essentially, we merged fff, aka DIY, and DIY+ which lived in diyplus
directory, into one called NERD. this is because the simplest DIY didn't
provide acceptable performance characteristics and they are both
suitable for a home-made DIY project anyway. hence, the "nerd" edition
aka "have fun playing around with it on your own".

it also makes things simpler. the other version we'll be making is KAFI.
there are no source files for it yet.

README.md

@@ -8,74 +8,28 @@ be sufficient but you might want to install
 [fasteners workbench](https://wiki.freecadweb.org/Fasteners_Workbench) if working on
 a design assembly.
 
-there are currently version kinds available, DIY (FFF) and DIY+, and possibly
+there are currently designs only for the [NERD edition](https://nakamochi.io/#nerd).
-third upcoming PRO. all versions have generated files like STL, STEP and DXF,
+KAFI version is coming later on. generated files like STL, STEP and DXF are attached
-attached to their respective releases on https://git.qcode.ch/nakamochi/3d/releases
+to their respective releases on https://git.qcode.ch/nakamochi/3d/releases
 
-## DIY version (FFF)
+## NERD edition
-
-fused filament fabrication design is suitable for 3D printing at home, located
-in the [fff](fff/) directory. the main file is in [fff/provcase.FCStd](fff/provcase.FCStd).
-"provcase" stands for "provisional case" because it was unclear at the time whether
-this would have been the final version.
-
-the project contains all the parts to make a full assembly with the following BOM:
-
-- [raspberry pi 4 model b][rpi4]
-- [waveshare 4.3inch DSI LCD][lcd]
-- [sandisk portable 1TB SSD sdssde30-1t00][ssd]
-- [joi-it armor "block" heatsink](https://joy-it.net/en/products/RB-AlucaseP4+07)
-- 90 degree angle [USB-C adapter][usbc90deg] 3.1 gen 2, 10Gbps
-- adhesive gasket/seal tape for LCD, 1mm thick, 10mm width, L370mm
-- 4 adhesive bottom pads
-- 4 screws M2.5 L16-18mm for heatsink mount
-- 4 spacers 2-4mm thick depending on the heatsink mount screws length
-- 3D-printed enclosure main box (look for `box.stl` in releases)
-- 3D-printed enclosure side cup (look for `sidecup.stl` in releases)
-
-all object constraints reference an embedded spreadsheet `params` to avoid 
-[topological naming problem](https://wiki.freecadweb.org/Topological_naming_problem).
-prefer constraining against datum objects instead of faces and other elements produced
-directly from sketches like pads.
-
-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 DIY case assembly](fff/provcase-assembly.png) | ![complete DIY build](fff/build.jpg) |
-
-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
-
-## DIY+ (plus)
 
 this version features a custom heatsink combined with the bottom plate, suitable
-for aluminium CNC milling. the construction provides a better heat dissipation
+for aluminium CNC milling. the construction provides a good heat dissipation,
-compared to the DIY (FFF) version.
+making an active cooling like a fan redundant.
 
 |    |    |
 | ---|--- |
-| ![DIY+ assembly unboxed exploded](diyplus/assembly_ub_exp.png) | ![DIY+ unboxed assembled](diyplus/assembly_ub_imp.png) |
+| ![NERD assembly unboxed exploded](nerd/assembly_ub_exp.png) | ![NERD unboxed assembled](nerd/assembly_ub_imp.png) |
-| ![DIY+ assembly](diyplus/assembly.png)                         | ![DIY+ assembly boxed](diyplus/assembly_boxed.png) |
+| ![NERD assembly](nerd/assembly.png)                         | ![NERD assembly boxed](nerd/assembly_boxed.png) |
 
-<img alt="DIY+ temperature during IBD" src="diyplus/ir_1166.jpg" align="right">
+<img alt="NERD temperature during IBD" src="nerd/ir_1166.jpg" align="right">
 the baseplate serving as a heatsink brings down the main CPU temperature
 from around 70℃  to 50℃. when measured externally while doing bitcoin initial
 blocks download, an IR camera showed around 36℃.
 
-the main file [diyplus.FCStd](diyplus/diyplus.FCStd) contains all parts
+the main file [nerd.FCStd](nerd/nerd.FCStd) contains all parts
-as well as the assembly. other files in the [diyplus](diyplus/) directory are
+as well as the assembly. a complete assembly BOM is as follows.
-exports from the main file, such as baseplate DXF, STL, STEP and drawings.
-
-a complete assembly BOM is as follows.
 
 **off the shelf parts**