feat: preview furniture placement

Issue: #359
User-Visible: yes

test/furniture.test.mjs

@@ -6,7 +6,8 @@ import assert from 'node:assert/strict';
 import {
   FURNITURE, FURNITURE_GROUPS, furnitureSymbol, furnitureOfGroup,
   furnitureDefaultCm, furniturePathD, furnitureGraphic, furnitureCorners, furnitureResize,
-  snapFurnitureToWall, cmToNorm, normToCm, clampFurnSize, clampFurnCm,
+  snapFurnitureToWall, resolveFurniturePlacement,
+  cmToNorm, normToCm, clampFurnSize, clampFurnCm,
   FURN_MIN_N, FURN_MIN_CM, FURN_MAX_CM, FURN_WALL_CELLS,
 } from '../test-build/furniture.js';
 import { roomEdges } from '../test-build/logic.js';
@@ -185,6 +186,38 @@ test('the default reach is six cells — thirty centimetres on a default plan',
   closeTo(GRID_PITCH * FURN_WALL_CELLS * (5 / GRID_PITCH), 30);  // cells x cell_cm
 });
 
+test('preview and commit share one deterministic furniture placement resolver', () => {
+  const input = {
+    symbol: 'sofa', widthCm: 180, depthCm: 90,
+    point: [300, 112], canvasW: 1000, canvasH: 1000,
+    cellCm: 5, gridPitch: GRID_PITCH, walls: EDGES,
+    wallReach: 30,
+  };
+  const preview = resolveFurniturePlacement(input);
+  const commit = resolveFurniturePlacement({ ...input });
+  assert.deepEqual(preview, commit);
+  assert.equal(preview.symbol, 'sofa');
+  closeTo((preview.x + preview.w / 2) * 1000, 300, GRID_PITCH);
+  closeTo((preview.y + preview.h / 2) * 1000, 100 + preview.h * 500);
+  assert.equal(preview.angle, 0);
+});
+
+test('the shared placement resolver supports Shift/free, canvas guards and unknown symbols', () => {
+  const base = {
+    symbol: 'sofa', widthCm: 180, depthCm: 90,
+    point: [300, 112], canvasW: 1000, canvasH: 1000,
+    cellCm: 5, gridPitch: GRID_PITCH, walls: EDGES,
+    wallReach: 30,
+  };
+  const free = resolveFurniturePlacement({ ...base, free: true });
+  closeTo((free.x + free.w / 2) * 1000, 300);
+  closeTo((free.y + free.h / 2) * 1000, 112);
+  const guarded = resolveFurniturePlacement({ ...base, point: [-1e9, -1e9], free: true });
+  assert.equal(guarded.x, -5000);
+  assert.equal(guarded.y, -5000);
+  assert.equal(resolveFurniturePlacement({ ...base, symbol: 'future_unknown_symbol' }), null);
+});
+
 // ------------------------------- the frame ----------------------------------
 
 test('the corners of an unrotated box are the box; a rotated one turns about its centre', () => {

test/golden-matrix.test.mjs

@@ -835,6 +835,17 @@ test('issue 159 goldens cover both palette levels and every plan-art compatibili
     new Set(['sofa', 'fridge', 'sofa_corner_right']));
 });
 
+test('issue 359 has one deterministic light-theme furniture placement preview golden', () => {
+  const scenario = GOLDEN_SCENARIOS.find((item) => item.id === 'furniture-placement-preview-light');
+  assert.ok(scenario);
+  assert.equal(scenario.mode, 'decor');
+  assert.equal(scenario.theme, 'light');
+  assert.equal(scenario.capture, 'stage');
+  assert.deepEqual(scenario.furniturePlacementPreview, {
+    symbol: 'sofa', widthCm: 180, depthCm: 90, pointer: [0.35, 0.90],
+  });
+});
+
 test('device dialog goldens expose the complete light-source controls at desktop and mobile widths', () => {
   const dialogs = GOLDEN_SCENARIOS.filter((scenario) => scenario.id.startsWith('device-dialog-'));
   assert.equal(dialogs.length, 3);