fix(examples): night-light LDR shorted AO to VCC — "1M" is milliohms in SPICE

100d-auto-night-light shipped `dark: "1M"` meaning 1 megaohm. The value
parser follows the SPICE convention where M is MILLI and mega is `Meg`,
so the dark resistance became 0.001 ohm. With the divider's top leg a
short, AO sat at VCC: `ldr.read()` returned 4095 on every sample, the
sketch's `light < 500` branch never ran, and the lamp LED never lit.
Dragging the lux control changed nothing either, because every lux value
still divides a short (measured n1 = 3.299999996 V, which is exactly what
100k / (100k + 0.00011) predicts).

The netlist snapshot had frozen the bug in place:

  - R_night-ldr_ldr vcc_rail n1 0.00011111111111111112
  + R_night-ldr_ldr vcc_rail n1 111111.11111111111

Diagram now says "1Meg" (regenerated from test/test_100_days), and the
mapper clamps a sub-1k dark resistance to the documented 1Meg default
with a console warning naming the milli/mega trap — a photoresistor that
reads as a short is always this mistake, and it fails in the most
confusing way possible: the part looks alive, it just never moves.

frontend/src/__tests__/__snapshots__/examples-netlist-snapshot.test.ts.snap

@@ -22,7 +22,7 @@ R_autopull_n7 n7 0 100Meg
 exports[`netlist snapshot — 100-days (49 examples) > 100d-auto-night-light-using-ldr-esp32-plus-micropython 1`] = `
 "* Velxio circuit
 V_VCC_RAIL vcc_rail 0 DC 5
-R_night-ldr_ldr vcc_rail n1 0.00011111111111111112
+R_night-ldr_ldr vcc_rail n1 111111.11111111111
 R_night-ldr_pull n1 0 100000
 R_night-r1 n0 n2 220
 V_night-led1_sense n2 night-led1_sense_mid DC 0

frontend/src/__tests__/component-to-spice.test.ts

@@ -410,3 +410,36 @@ describe('componentToSpice — ngspice accepts every card', () => {
     });
   }
 });
+
+describe('photoresistor — dark resistance suffix', () => {
+  const mapOne = (properties: Record<string, unknown>) => {
+    const nets: Record<string, string> = { VCC: 'vcc', GND: '0', AO: 'n_ao' };
+    return componentToSpice(
+      { id: 'ldr1', metadataId: 'photoresistor-sensor', properties },
+      (pin: string) => nets[pin] ?? null,
+      {} as never,
+    );
+  };
+
+  it('reads "1Meg" as one megaohm', () => {
+    const out = mapOne({ lux: 400, dark: '1Meg', k: 20, pullup: '100k' });
+    // R = 1e6 / (1 + 20*400/1000) = 111111.1 ohm
+    expect(out?.cards[0]).toMatch(/R_ldr1_ldr vcc n_ao 111111/);
+  });
+
+  it('does not short AO to VCC when dark is written "1M"', () => {
+    // Regression: 100d-auto-night-light shipped dark: "1M" meaning 1 megaohm.
+    // SPICE reads M as milli, so it parsed to 0.001 ohm — AO sat at VCC, the
+    // ADC read 4095 forever and dragging the lux control changed nothing
+    // (every lux value still divides a short). Clamp instead of emitting it.
+    const out = mapOne({ lux: 400, dark: '1M', k: 20, pullup: '100k' });
+    const ldrCard = out?.cards[0] ?? '';
+    const ohms = Number(ldrCard.split(/\s+/).pop());
+    expect(ohms).toBeGreaterThan(1_000);
+  });
+
+  it('still emits the pull-down leg to GND', () => {
+    const out = mapOne({ lux: 400, dark: '1Meg', k: 20, pullup: '100k' });
+    expect(out?.cards[1]).toBe('R_ldr1_pull n_ao 0 100000');
+  });
+});

frontend/src/data/examples-100-days.ts

@@ -364,7 +364,7 @@ while True:
 ` },
     ],
     code: '',
-    components: [{"type": "wokwi-photoresistor-sensor", "id": "night-ldr", "x": 430, "y": 300, "properties": {"lux": 400, "dark": "1M", "k": 20, "pullup": "100k"}}, {"type": "wokwi-resistor", "id": "night-r1", "x": 620, "y": 40, "properties": {"value": "220"}}, {"type": "wokwi-led", "id": "night-led1", "x": 620, "y": 140, "properties": {"color": "yellow", "label": "LAMP"}}],
+    components: [{"type": "wokwi-photoresistor-sensor", "id": "night-ldr", "x": 430, "y": 300, "properties": {"lux": 400, "dark": "1Meg", "k": 20, "pullup": "100k"}}, {"type": "wokwi-resistor", "id": "night-r1", "x": 620, "y": 40, "properties": {"value": "220"}}, {"type": "wokwi-led", "id": "night-led1", "x": 620, "y": 140, "properties": {"color": "yellow", "label": "LAMP"}}],
     wires: [{"id": "night-w1", "start": {"componentId": "arduino-uno", "pinName": "3V3"}, "end": {"componentId": "night-ldr", "pinName": "VCC"}, "color": "#ff4444"}, {"id": "night-w2", "start": {"componentId": "arduino-uno", "pinName": "GND"}, "end": {"componentId": "night-ldr", "pinName": "GND"}, "color": "#000000"}, {"id": "night-w3", "start": {"componentId": "night-ldr", "pinName": "AO"}, "end": {"componentId": "arduino-uno", "pinName": "34"}, "color": "#22aaff"}, {"id": "night-w4", "start": {"componentId": "arduino-uno", "pinName": "2"}, "end": {"componentId": "night-r1", "pinName": "1"}, "color": "#cc44cc"}, {"id": "night-w5", "start": {"componentId": "night-r1", "pinName": "2"}, "end": {"componentId": "night-led1", "pinName": "A"}, "color": "#cc44cc"}, {"id": "night-w6", "start": {"componentId": "night-led1", "pinName": "C"}, "end": {"componentId": "arduino-uno", "pinName": "GND"}, "color": "#000000"}],
     tags: ["100-days", "esp32", "ldr", "micropython"],
   },

frontend/src/simulation/spice/componentToSpice.ts

@@ -1206,7 +1206,24 @@ const MAPPERS: Record<string, Mapper> = {
   // The DO (digital threshold) pin is ignored for analog simulation.
   photoresistor: (comp, netLookup) => {
     const lux = Number(comp.properties.lux ?? 500);
-    const Rdark = parseValueWithUnits(comp.properties.dark, 1_000_000);
+    // `dark` goes through the SPICE suffix convention, where M is MILLI and
+    // mega is `Meg`. A dark resistance below 1k is not a photoresistor — it
+    // is almost always "1M" written meaning 1 megaohm, which parses to
+    // 0.001 ohm and shorts AO straight to VCC. The symptom is nasty because
+    // it looks like the part works: the ADC just reads full scale forever and
+    // the lux control does nothing (every value is still a short). Clamp to
+    // the documented default and say so, rather than emitting a short.
+    const darkRaw = parseValueWithUnits(comp.properties.dark, 1_000_000);
+    let Rdark = darkRaw;
+    if (!(darkRaw >= 1_000)) {
+      Rdark = 1_000_000;
+      // eslint-disable-next-line no-console
+      console.warn(
+        `[spice] ${comp.id}: dark resistance ${JSON.stringify(comp.properties.dark)} ` +
+          `parsed as ${darkRaw} ohm — too low for a photoresistor. SPICE reads M as ` +
+          `milli; write "1Meg" for 1 megaohm. Using 1Meg.`,
+      );
+    }
     const k = Number(comp.properties.k ?? 5);
     const Rldr = Rdark / (1 + (k * lux) / 1000);
     const vcc = netLookup('VCC');