fix: make plan visuals grid-scale invariant

Issue: #239
User-Visible: yes

test/grid-scale.test.mjs

@@ -0,0 +1,41 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  GRID_IMPERIAL_CELL_CM,
+  GRID_VISUAL_REFERENCE_CELL_CM,
+  gridCellFieldToCm,
+  gridCellFieldValue,
+  gridVisualScale,
+  gridVisualUnits,
+  newSpaceCellCm,
+} from '../test-build/grid-scale.js';
+
+test('visual scale preserves the 5 cm reference and follows physical equivalence', () => {
+  assert.equal(GRID_VISUAL_REFERENCE_CELL_CM, 5);
+  assert.equal(gridVisualScale(5), 1);
+  for (const cellCm of [1, 2.54, 10, 25]) {
+    assert.equal(gridVisualScale(cellCm), 5 / cellCm);
+    assert.equal(gridVisualUnits(2.5, cellCm), 2.5 * (5 / cellCm));
+  }
+});
+
+test('invalid visual-scale inputs retain the legacy 5 cm appearance', () => {
+  for (const value of [NaN, Infinity, -Infinity, 0, -1, undefined, null, '', '1', 'not-a-number']) {
+    assert.equal(gridVisualScale(value), 1);
+  }
+});
+
+test('new-space defaults use one centimetre or exactly one inch', () => {
+  assert.equal(newSpaceCellCm(false), 1);
+  assert.equal(newSpaceCellCm(true), GRID_IMPERIAL_CELL_CM);
+  assert.equal(GRID_IMPERIAL_CELL_CM, 2.54);
+});
+
+test('imperial field projection is readable while canonical values stay available', () => {
+  assert.equal(gridCellFieldValue(2.54, true), '1');
+  assert.equal(gridCellFieldValue(1, false), '1');
+  assert.equal(gridCellFieldValue(5, true), '1.968504');
+  assert.equal(gridCellFieldToCm(1, true), 2.54);
+  assert.equal(gridCellFieldToCm(1.25, true), 3.175);
+  assert.equal(gridCellFieldToCm(1.25, false), 1.25);
+});