site(play): ship explicit spec for the rides example

The "Bike rides by hour" chip had `spec: null` in the manifest — picking
it only loaded the CSV and left whatever spec the editor already had.
After picking, say, Quarterly revenue first, switching to rides left
the editor on the quarterly spec (`field: "quarter"`), which then
errored against rides.csv:

  Layer 0 encoding.x references field "quarter" which is not in the
  schema. Available columns: [hour, fare, rides]

Fix: ship `site/play/examples/rides.spec.json` and reference it from
the manifest. Every CSV-bearing example now ships an explicit spec —
no entry in the gallery still depends on the editor's prior state.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: seanhanca

site/play/examples/index.json

@@ -5,7 +5,7 @@
     "name": "Bike rides by hour",
     "description": "The original playground example — 12-row CSV, bar chart of hourly rides.",
     "csv": "examples/rides.csv",
-    "spec": null
+    "spec": "examples/rides.spec.json"
   },
   {
     "id": "quarterly-revenue",

site/play/examples/rides.spec.json

@@ -0,0 +1,13 @@
+{
+  "version": "glyph/0.1",
+  "title": "Bike rides by hour",
+  "layers": [
+    {
+      "mark": "bar",
+      "encoding": {
+        "x": { "field": "hour", "type": "ordinal" },
+        "y": { "field": "rides", "type": "quantitative" }
+      }
+    }
+  ]
+}