Add a heatmap spot size slider to the Active Fires row

A ½×–4× slider (log2 axis, 5 % steps, default 1×) under the intensity
filter sets how large far-zoom heat spots are. It scales the grid, not
the blur: 2× is a 1024 × 512 grid with the same one-pixel blur, so each
spot grows exactly and weak lone fires don't fade as they widen. A change
rebuilds only the heatmap. Share links keep it as w.s (percent, 50–400).
The layer row now takes a list of sliders.

src/data/layerState.test.mjs

@@ -30,6 +30,11 @@ import { stampInitialShareGesture } from '../navigationPolicy.js';
 import { MAX_SINCE_DAYS } from '../layers/streetLevel/policy.js';
 import { SINCE_STOPS } from '../ui/streetLevelPresentation.js';
 import { FIRMS_CONFIDENCE_MODES, MIN_FRP_MW_MAX } from './firmsIntensity.js';
+import {
+  HEAT_SPOT_SIZE_DEFAULT,
+  HEAT_SPOT_SIZE_MAX,
+  HEAT_SPOT_SIZE_MIN,
+} from './firmsHeatRaster.js';
 
 function deferred() {
   let resolve;
@@ -3019,9 +3024,14 @@ test('Active Fires: the intensity filter rides share token w and round-trips', (
   assert.deepEqual(state.options['local-firms'], {
     minFrp: 0,
     confidence: 'all',
+    spotSize: 100,
   });
   state.enabledLayerIds = ['local-firms'];
-  state.options['local-firms'] = { minFrp: 20, confidence: 'high' };
+  state.options['local-firms'] = {
+    minFrp: 20,
+    confidence: 'high',
+    spotSize: 250,
+  };
   const encoded = encode(state);
   assert.equal(new URLSearchParams(encoded).get('l'), 'w');
   // Other owners write their own explicit defaults into `lo` too (flights 3D,
@@ -3029,10 +3039,12 @@ test('Active Fires: the intensity filter rides share token w and round-trips', (
   const assignments = new URLSearchParams(encoded).get('lo').split('_');
   assert.ok(assignments.includes('w.f.20'), assignments.join(' '));
   assert.ok(assignments.includes('w.c.h'), assignments.join(' '));
+  assert.ok(assignments.includes('w.s.250'), assignments.join(' '));
   const decoded = decodeLayerStateParams(new URLSearchParams(encoded));
   assert.deepEqual(decoded.options['local-firms'], {
     minFrp: 20,
     confidence: 'high',
+    spotSize: 250,
   });
   assert.deepEqual(
     parseStoredLayerState(serializeStoredLayerState(state)).options[
@@ -3050,7 +3062,11 @@ test('Active Fires: the intensity filter rides share token w and round-trips', (
       .filter((assignment) => assignment.startsWith('w.')).length,
     0,
   );
-  assert.deepEqual(firmsOptions(''), { minFrp: 0, confidence: 'all' });
+  assert.deepEqual(firmsOptions(''), {
+    minFrp: 0,
+    confidence: 'all',
+    spotSize: 100,
+  });
 });
 
 test('Active Fires: the codec accepts whole megawatts in bounds and the three confidence codes', () => {
@@ -3078,5 +3094,17 @@ test('Active Fires: the codec accepts whole megawatts in bounds and the three co
   assert.deepEqual(firmsOptions('w.c.n_w.f.5'), {
     minFrp: 5,
     confidence: 'nominal',
+    spotSize: 100,
   });
 });
+
+test('Active Fires: the codec accepts whole-percent spot sizes from half to four times', () => {
+  assert.equal(HEAT_SPOT_SIZE_MIN, 50);
+  assert.equal(HEAT_SPOT_SIZE_MAX, 400);
+  assert.equal(HEAT_SPOT_SIZE_DEFAULT, 100);
+  assert.equal(firmsOptions('w.s.50').spotSize, 50);
+  assert.equal(firmsOptions('w.s.400').spotSize, 400);
+  assert.equal(firmsOptions('w.s.45').spotSize, 100, 'out of bounds → default');
+  assert.equal(firmsOptions('w.s.401').spotSize, 100);
+  assert.equal(firmsOptions('w.s.x').spotSize, 100);
+});