feat(iso): highlight walkable adjacent cells (openings vs wall-gaps)

The Kenney iso wall tiles are single-cell segments that leave triangular gaps between slabs, which
read as doorways but are solid wall. Rather than chase perfect wall continuity, flag the cells the
player can actually step to: a soft pulsing cyan diamond outline on each orthogonally-adjacent
walkable+explored floor cell. A reliable "you can go here" cue regardless of wall rendering.

html/js/render/isoRenderer.js

@@ -83,6 +83,20 @@
         return null;
     };
 
+    // A stroked iso diamond outline — used to flag the floor cells the player can actually step to,
+    // so ambiguous wall gaps are never mistaken for openings.
+    IsoRenderer.prototype._diamondOutline = function (cx, cy, color) {
+        var ctx = this.ctx, hw = this.tileW / 2, hh = this.tileH / 2;
+        ctx.save();
+        ctx.beginPath();
+        ctx.moveTo(cx, cy - hh); ctx.lineTo(cx + hw, cy); ctx.lineTo(cx, cy + hh); ctx.lineTo(cx - hw, cy);
+        ctx.closePath();
+        ctx.lineWidth = 2.5;
+        ctx.strokeStyle = color;
+        ctx.stroke();
+        ctx.restore();
+    };
+
     // Draw a text glyph centered at a projected cell (feature fallback when no pack tile fits).
     IsoRenderer.prototype._glyph = function (cx, cy, ch, color) {
         var ctx = this.ctx;
@@ -402,6 +416,25 @@
                 }
             }
         }
+
+        // Walkability affordance: the iso wall tiles leave ambiguous gaps that read as doorways, so
+        // flag the floor cells the player can actually step to (orthogonally adjacent, walkable +
+        // explored) with a soft pulsing outline — a reliable "you can go here" cue independent of how
+        // the walls render.
+        if (this._plx != null) {
+            var pulse = 0.30 + Math.sin(now / 320) * 0.18;
+            var nbrs = [[0, -1], [0, 1], [-1, 0], [1, 0]];
+            for (var ni = 0; ni < nbrs.length; ni++) {
+                var nx = this._plx + nbrs[ni][0], ny = this._ply + nbrs[ni][1];
+                if (ny < 0 || nx < 0 || !scene.grid[ny] || nx >= scene.cols) continue;
+                var nk = scene.grid[ny][nx];
+                if (nk === 'wall' || nk === 'dark' || nk === 'torch' || nk === undefined) continue;
+                var np = this._project(nx, ny, originX, originY);
+                this._diamondOutline(np.x, np.y + this.tileH * 0.5, 'rgba(130,220,255,' + pulse + ')');
+            }
+            this._animating = true; // keep the pulse animating between game updates
+        }
+
         this._vignette();
         if (this._animating) this._invalidate();
     };