active trapdoor demo (#976)

Towards #1088
![image](https://github.com/swarm-game/swarm/assets/261693/c48bd24b-23b8-4165-8670-c105301f04c4)

data/scenarios/00-ORDER.txt

@@ -7,3 +7,4 @@ Fun
 Speedruns
 Testing
 Vignettes
+Mechanics

data/scenarios/Mechanics/00-ORDER.txt

@@ -0,0 +1 @@
+active-trapdoor.yaml

data/scenarios/Mechanics/_active-trapdoor/gate.sw

@@ -0,0 +1,31 @@
+/**
+This mechanic operates by polling.
+Note that the polling is not naturally "throttled" since
+the 'whereami' command is intangible (has zero duration).
+So we insert 'wait' commands to be performance-friendly.
+*/
+def waitAndGate = \myCoords. \armed.
+  basePos <- as base {whereami};
+
+  if (basePos == myCoords) {
+    if armed {} {
+      // Warn the player that they have tripped the trap
+      say "ka-chunk"
+    };
+    // The trapdoor is now armed. Wait to spring it.
+    wait 1; // Throttle the polling
+    waitAndGate myCoords true;
+  } {
+    if (armed) {
+      place "boulder";
+      // recursion ends
+    } {
+      // Wait to arm the trapdoor
+      wait 1; // Throttle the polling
+      waitAndGate myCoords false
+    };
+  };
+  end;
+
+myPos <- whereami;
+waitAndGate myPos false;

data/scenarios/Mechanics/_active-trapdoor/solution.sw

@@ -0,0 +1,4 @@
+def doN = \n. \f. if (n > 0) {f; doN (n - 1) f} {}; end;
+
+doN 10 move;
+grab;

data/scenarios/Mechanics/active-trapdoor.yaml

@@ -0,0 +1,81 @@
+version: 1
+name: Active trapdoor
+author: Karl Ostmo
+description: |
+  This gate closes once you have passed.
+
+  This is an "active" trapdoor in the sense that a system robot
+  continuously monitors its status and operates it.
+  It is in contrast with a sokoban-style trapdoor
+  that is "passive" in its operation.
+creative: false
+objectives:
+  - goal:
+      - Grab the flower.
+    condition: |
+      as base {has "flower"}
+robots:
+  - name: base
+    display:
+      char: 'ω'
+      attr: robot
+    dir: [0, 1]
+    devices:
+      - treads
+      - 3D printer
+      - ADT calculator
+      - branch predictor
+      - clock
+      - comparator
+      - counter
+      - dictionary
+      - grabber
+      - hearing aid
+      - lambda
+      - logger
+      - mirror
+      - net
+      - scanner
+      - strange loop
+      - string
+      - workbench
+    inventory:
+      - [1, treads]
+  - name: gate
+    system: true
+    dir: [0, 1]
+    display:
+      invisible: true
+    inventory:
+      - [1, boulder]
+    program: |
+      run "scenarios/Mechanics/_active-trapdoor/gate.sw"
+solution: |
+  run "scenarios/Mechanics/_active-trapdoor/solution.sw"
+known: [water, boulder, flower]
+seed: 0
+world:
+  default: [stone, water]
+  upperleft: [0, 0]
+  offset: false
+  palette:
+    '@': [stone, boulder]
+    '.': [grass]
+    G: [stone, null, gate]
+    Ω: [grass, null, base]
+    f: [grass, flower]
+  map: |
+    .....
+    ..f..
+    .....
+    @@.@@
+    .@.@.
+    .@.@.
+    .@G@.
+    .@.@.
+    .@.@.
+    @@.@@
+    .....
+    ..Ω..
+    .....
+  

test/integration/Main.hs

@@ -217,6 +217,10 @@ testScenarioSolution _ci _em =
             , testSolution Default "Challenges/Sokoban/Gadgets/one-way.yaml"
             , testSolution Default "Challenges/Sokoban/Simple/trapdoor.yaml"
             ]
+        , testGroup
+            "Mechanics"
+            [ testSolution Default "Mechanics/active-trapdoor.yaml"
+            ]
         ]
     , testGroup
         "Regression tests"