style(model): align every sample layout to the 20px grid

Snap all sample coordinates to multiples of 20 so they sit on the
editor grid and read cleanly. Computed midpoint() valves had to land
on grid points too, so a few endpoints were nudged (e.g. escalation
arsenals 300->280, limits-to-growth source -300->-280). Population is
re-laid as a grid cascade so its links no longer overlap the pipes.

src/model/samples.ts

@@ -68,9 +68,9 @@ function model(name: string, nodes: ModelNode[], infoLinks: InformationLink[]):
  * feedback: it just shows the substrate (Stock, Flow, Source, Sink).
  */
 function bathtub(): Model {
-  const source = makeCloud({ x: -278, y: -18 })
+  const source = makeCloud({ x: -280, y: 0 })
-  const water = makeStock({ x: -48, y: -20 }, "Water")
+  const water = makeStock({ x: 0, y: 0 }, "Water")
-  const sink = makeCloud({ x: 242, y: -18 })
+  const sink = makeCloud({ x: 280, y: 0 })
   const filling = makeFlow(
     midpoint(source.position, water.position),
     "filling",
@@ -90,8 +90,8 @@ function savings(): Model {
   // Source up-left, Balance down-right: the interest valve lands at their
   // midpoint (above Balance), so the `Balance → interest` link arcs back up as a
   // visible Reinforcing loop instead of overlapping the inflow pipe.
-  const source = makeCloud({ x: -250, y: -70 })
+  const source = makeCloud({ x: -240, y: -80 })
-  const balance = makeStock({ x: 110, y: 30 }, "Balance")
+  const balance = makeStock({ x: 120, y: 40 }, "Balance")
   const interest = makeFlow(
     midpoint(source.position, balance.position),
     "interest",
@@ -108,15 +108,15 @@ function savings(): Model {
  * (outflow) → Coffee has one `−` → Balancing: it settles toward room temperature.
  */
 function coffee(): Model {
-  const coffee = makeStock({ x: -200, y: -10 }, "Coffee")
+  const coffee = makeStock({ x: -200, y: 0 }, "Coffee")
-  const sink = makeCloud({ x: 190, y: -8 })
+  const sink = makeCloud({ x: 200, y: 0 })
   const heatLoss = makeFlow(
     midpoint(coffee.position, sink.position),
     "heat loss",
     coffee.id,
     sink.id,
   )
-  const room = makeConverter({ x: -29, y: -150 }, "room temperature")
+  const room = makeConverter({ x: 0, y: -160 }, "room temperature")
   return model(
     "Coffee cooling",
     [coffee, sink, heatLoss, room],
@@ -131,18 +131,17 @@ function coffee(): Model {
  * births (`+`), life expectancy lowers deaths (`−`).
  */
 function population(): Model {
-  const source = makeCloud({ x: -360, y: -8 })
+  // A grid-aligned cascade (20px grid): Source and both Converters stack in the left
-  const people = makeStock({ x: -48, y: -10 }, "Population")
+  // column, the Stock sits at the centre, and births → Population → deaths step down
-  const sink = makeCloud({ x: 250, y: -8 })
+  // toward the Sink — so every Information Link lands in open space, not on a pipe.
-  const births = makeFlow(
+  // Valves are placed by hand, not at the midpoint, to hold the steps.
-    midpoint(source.position, people.position),
+  const source = makeCloud({ x: -360, y: -240 })
-    "births",
+  const fertility = makeConverter({ x: -360, y: -40 }, "fertility")
-    source.id,
+  const lifeExpectancy = makeConverter({ x: -360, y: 240 }, "life expectancy")
-    people.id,
+  const people = makeStock({ x: 0, y: 0 }, "Population")
-  )
+  const births = makeFlow({ x: -160, y: -160 }, "births", source.id, people.id)
-  const deaths = makeFlow(midpoint(people.position, sink.position), "deaths", people.id, sink.id)
+  const sink = makeCloud({ x: 360, y: 240 })
-  const fertility = makeConverter({ x: -204, y: -150 }, "fertility")
+  const deaths = makeFlow({ x: 160, y: 160 }, "deaths", people.id, sink.id)
-  const lifeExpectancy = makeConverter({ x: 101, y: -150 }, "life expectancy")
   return model(
     "Population",
     [source, people, sink, births, deaths, fertility, lifeExpectancy],
@@ -164,13 +163,13 @@ function population(): Model {
  * the balancing loop without sitting on any cycle.
  */
 function limitsToGrowth(): Model {
-  const source = makeCloud({ x: -300, y: 0 })
+  const source = makeCloud({ x: -280, y: 0 })
   const yeast = makeStock({ x: 40, y: 0 }, "Yeast")
   const growth = makeFlow(midpoint(source.position, yeast.position), "growth", source.id, yeast.id)
   // crowding rides above the pipe; carrying capacity sits to its right so the
   // `capacity → crowding` link is a short horizontal hop along the top.
-  const crowding = makeConverter({ x: -40, y: -150 }, "crowding")
+  const crowding = makeConverter({ x: -40, y: -160 }, "crowding")
-  const capacity = makeConverter({ x: 170, y: -150 }, "carrying capacity")
+  const capacity = makeConverter({ x: 160, y: -160 }, "carrying capacity")
   return model(
     "Limits to growth",
     [source, yeast, growth, crowding, capacity],
@@ -275,7 +274,7 @@ function epidemic(): Model {
     infected.id,
     recovered.id,
   )
-  const infectivity = makeConverter({ x: -140, y: -150 }, "infectivity")
+  const infectivity = makeConverter({ x: -140, y: -160 }, "infectivity")
   return model(
     "Epidemic",
     [susceptible, infected, recovered, infection, recovery, infectivity],
@@ -304,7 +303,7 @@ function tragedyOfTheCommons(): Model {
   // Two symmetric herds: cattle enter from a Source on the outside, grass leaves
   // the Pasture downward to a Sink. The two `Pasture → growth` links are the weak
   // brake the trap overruns.
-  const sourceA = makeCloud({ x: -620, y: 0 })
+  const sourceA = makeCloud({ x: -640, y: 0 })
   const herdA = makeStock({ x: -360, y: 0 }, "Herd A")
   const growthA = makeFlow(
     midpoint(sourceA.position, herdA.position),
@@ -312,14 +311,14 @@ function tragedyOfTheCommons(): Model {
     sourceA.id,
     herdA.id,
   )
-  const sinkA = makeCloud({ x: -180, y: 220 })
+  const sinkA = makeCloud({ x: -200, y: 240 })
   const grazingA = makeFlow(
     midpoint(pasture.position, sinkA.position),
     "grazing A",
     pasture.id,
     sinkA.id,
   )
-  const sourceB = makeCloud({ x: 620, y: 0 })
+  const sourceB = makeCloud({ x: 640, y: 0 })
   const herdB = makeStock({ x: 360, y: 0 }, "Herd B")
   const growthB = makeFlow(
     midpoint(sourceB.position, herdB.position),
@@ -327,7 +326,7 @@ function tragedyOfTheCommons(): Model {
     sourceB.id,
     herdB.id,
   )
-  const sinkB = makeCloud({ x: 180, y: 220 })
+  const sinkB = makeCloud({ x: 200, y: 240 })
   const grazingB = makeFlow(
     midpoint(pasture.position, sinkB.position),
     "grazing B",
@@ -362,7 +361,7 @@ function escalation(): Model {
   // Source to its arsenal. The two cross-coupling links span the open centre and
   // cross there, where the R badge lands, so the whole loop reads at a glance.
   const blueSource = makeCloud({ x: -560, y: -120 })
-  const blueArsenal = makeStock({ x: 300, y: -120 }, "Blue arsenal")
+  const blueArsenal = makeStock({ x: 280, y: -120 }, "Blue arsenal")
   const blueBuildup = makeFlow(
     midpoint(blueSource.position, blueArsenal.position),
     "Blue buildup",
@@ -370,7 +369,7 @@ function escalation(): Model {
     blueArsenal.id,
   )
   const redSource = makeCloud({ x: -560, y: 120 })
-  const redArsenal = makeStock({ x: 300, y: 120 }, "Red arsenal")
+  const redArsenal = makeStock({ x: 280, y: 120 }, "Red arsenal")
   const redBuildup = makeFlow(
     midpoint(redSource.position, redArsenal.position),
     "Red buildup",