Add latches to the lid

README.md

@@ -1,12 +1,13 @@
 # Wargame Accessory Box & Dice Tray
 
 A parametric OpenSCAD box for carrying tabletop wargaming accessories.
-Three printable parts:
+Four printable parts:
 
 | Part     | Purpose                                                            |
 |----------|--------------------------------------------------------------------|
 | `box`    | Open tray. With the insert removed it doubles as a dice tray.      |
-| `lid`    | Fully detachable friction-fit lid (no hinge to get in the way).    |
+| `lid`    | Fully detachable lid, held shut by snap latches hinged on filament pins (or a classic friction fit). |
+| `latch`  | One-piece snap latch — print two per latch position.               |
 | `insert` | Removable compartment tray for tokens, measuring tapes, etc.       |
 
 ![Assembly](docs/assembly.png)
@@ -19,14 +20,50 @@ the free space in your backpack — they are **outer** dimensions and
 measured space exactly.
 
 Pick what to render with the `part` variable (or the customizer):
-`assembly` (exploded preview), `box`, `lid`, `insert`, `print`.
+`assembly` (exploded preview), `box`, `lid`, `insert`, `latch`, `print`.
 
 Or export everything from the command line:
 
 ```sh
-make            # exports stl/box.stl, stl/lid.stl, stl/insert.stl
+make            # exports stl/box.stl, stl/lid.stl, stl/insert.stl, stl/latch.stl
 ```
 
+## The latch lid
+
+By default (`lid_style = "latch"`) the lid is held shut by snap
+latches that hinge on short pieces of filament:
+
+* The **lid** carries pairs of lugs along its front and back edges.
+* Each **latch** is a single printed part with a pivot barrel that
+  fits between a lug pair. Its window snaps over a chamfered catch
+  bar on the box wall; pull the flared tip outward to open.
+* To assemble one latch: cut a ~23 mm piece of 1.75 mm filament,
+  slide it through lug → latch barrel → lug, and fix it with a drop
+  of glue **on the outer lug holes only** — the lug holes are sized
+  tight and the barrel hole loose, so the latch keeps pivoting
+  freely. Trim the pin flush.
+
+Tuning knobs (all per side unless noted):
+
+* `latches_per_side` — how many latches on each long wall
+  (default 2, evenly spread).
+* `catch_proud` — how far the catch bar sticks out = how hard the
+  snap is (default 1.4 mm).
+* `latch_play` — vertical play between latch and catch bar
+  (default 0.1 mm). Use a small negative value to preload the lid
+  shut.
+* `filament_d` — hinge pin diameter, in case you want to use
+  2.85 mm filament or a piece of wire instead.
+
+Note: lugs and latches stick out about 7 mm beyond the box on the
+front and back, so allow `total_y + 14` of space in the backpack.
+With latches doing the holding, a looser lip (`lid_clearance` around
+0.3–0.4) makes the lid pleasant to take off.
+
+Prefer the original hinge-free push-fit lid? Set
+`lid_style = "friction"` and you get the old behaviour, including
+the thumb grooves for prying the lid off.
+
 ## Configuring the insert layout
 
 Compartments are defined by the `layout` list, one entry per row
@@ -55,10 +92,13 @@ of the automatic height and print two with different layouts.
 ## How the parts work together
 
 * The **lid** is a flat plate with a lip that plugs into the box opening.
-  `lid_clearance` (default 0.2 mm per side) controls the friction fit —
-  print a test fit and adjust for your printer/material. Shallow thumb
-  grooves on the box's front and back walls let you pry the lid off; they
-  do **not** pierce the wall, so the dice tray stays fully closed.
+  With the latch lid the lip just aligns the lid (and keeps tokens in
+  their compartments); with `lid_style = "friction"` it carries the
+  whole fit. `lid_clearance` (default 0.2 mm per side) controls how
+  snug it is — print a test fit and adjust for your printer/material.
+  In friction mode, shallow thumb grooves on the box's front and back
+  walls let you pry the lid off; they do **not** pierce the wall, so
+  the dice tray stays fully closed.
 * The **insert** sits below the lid's lip, so the lip also keeps loose
   tokens from jumping compartments in the bag. Finger notches on its
   short walls let you lift it straight out; then the empty box is your
@@ -68,7 +108,9 @@ of the automatic height and print two with different layouts.
 ## Printing notes
 
 * No supports needed for any part. Print all parts flat side down
-  (the lid prints plate-down, lip up).
+  (the lid prints plate-down, lip and lugs up). The latch prints
+  standing on its flat side — that gives a clean pivot hole and puts
+  the layer lines along the strip, where the snap stress is.
 * PLA or PETG, 2–3 perimeters, ~10 % infill (the parts are mostly walls).
 * If the lid is too tight/loose, tune `lid_clearance` in steps of 0.05 mm.
   Same for the insert with `insert_clearance` (looser is fine there —