fix: make plan visuals grid-scale invariant

Issue: #239
User-Visible: yes

src/grid-scale.ts

@@ -0,0 +1,33 @@
+/** Historical plan-cell size whose raw SVG constants define the visual baseline. */
+export const GRID_VISUAL_REFERENCE_CELL_CM = 5;
+
+/** Canonical centimetres in one imperial grid cell (one inch). */
+export const GRID_IMPERIAL_CELL_CM = 2.54;
+
+/** Scale a legacy SVG visual unit without changing physical or screen-space sizes. */
+export function gridVisualScale(cellCm: unknown): number {
+  const value = typeof cellCm === 'number' ? cellCm : NaN;
+  if (!Number.isFinite(value) || value <= 0) return 1;
+  if (value === GRID_VISUAL_REFERENCE_CELL_CM) return 1;
+  return GRID_VISUAL_REFERENCE_CELL_CM / value;
+}
+
+export function gridVisualUnits(baseUnits: number, cellCm: unknown): number {
+  return baseUnits * gridVisualScale(cellCm);
+}
+
+/** Default only for newly-created spaces. Read compatibility keeps its own 5 cm fallback. */
+export function newSpaceCellCm(imperial: boolean): number {
+  return imperial ? GRID_IMPERIAL_CELL_CM : 1;
+}
+
+/** Human-sized field projection; the canonical draft remains separate and lossless. */
+export function gridCellFieldValue(cellCm: number, imperial: boolean): string {
+  if (!imperial) return String(cellCm);
+  const inches = cellCm / GRID_IMPERIAL_CELL_CM;
+  return String(Math.round(inches * 1_000_000) / 1_000_000);
+}
+
+export function gridCellFieldToCm(value: number, imperial: boolean): number {
+  return imperial ? value * GRID_IMPERIAL_CELL_CM : value;
+}