Opening moves become row-honest: ← → within the row, clamped at edges

Eric asked what the ↑↓ buttons did — the honest answer was "reorder a
flat list that no longer exists, and fragment your wall diagram if
pushed across a row boundary" (A1, B1, A2 renders as THREE rows under
the letter-grouped layout). The useful half survives as ← → "move
within its row"; the endpoint clamps at row-letter boundaries so the
diagram can't fragment. Tests pin both the clamp and the legitimate
within-row swap; the acceptance flow's reorder step updated to match
the new semantics.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jXZFSTZQKzAC8fqpWSz9g

data/furniture.json

@@ -22,7 +22,7 @@
         {
           "id": "library-kallax-b1",
           "label": "B1",
-          "zone": "cubes",
+          "zone": "party",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4
@@ -30,7 +30,7 @@
         {
           "id": "library-kallax-b2",
           "label": "B2",
-          "zone": "cubes",
+          "zone": "party",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4
@@ -38,7 +38,7 @@
         {
           "id": "library-kallax-b3",
           "label": "B3",
-          "zone": "cubes",
+          "zone": "party",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4
@@ -46,7 +46,7 @@
         {
           "id": "library-kallax-b4",
           "label": "B4",
-          "zone": "cubes",
+          "zone": "two-player",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4
@@ -54,7 +54,7 @@
         {
           "id": "library-kallax-c1",
           "label": "C1",
-          "zone": "cubes",
+          "zone": "general",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4
@@ -62,7 +62,7 @@
         {
           "id": "library-kallax-c2",
           "label": "C2",
-          "zone": "cubes",
+          "zone": "general",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4
@@ -70,7 +70,7 @@
         {
           "id": "library-kallax-c3",
           "label": "C3",
-          "zone": "cubes",
+          "zone": "general",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4
@@ -78,7 +78,7 @@
         {
           "id": "library-kallax-c4",
           "label": "C4",
-          "zone": "cubes",
+          "zone": "general",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4
@@ -86,7 +86,7 @@
         {
           "id": "library-kallax-d1",
           "label": "D1",
-          "zone": "cubes",
+          "zone": "general",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4
@@ -94,7 +94,7 @@
         {
           "id": "library-kallax-d2",
           "label": "D2",
-          "zone": "cubes",
+          "zone": "vintage wargames",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4
@@ -102,7 +102,7 @@
         {
           "id": "library-kallax-d3",
           "label": "D3",
-          "zone": "cubes",
+          "zone": "heavy",
           "width_in": 13.25,
           "height_in": 13.25,
           "depth_in": 15.4