Add inverted-axis and aspect-ratio sins; rename slugs

Writes two sins off the backlog. The inverted y-axis is the egregious
one: the fill hangs from the top and the line descends while the values
climb, with every element individually defensible. Aspect ratio is the
quiet one: same twelve numbers, same y-range, only the shape of the box
changes.

Adds Cleveland, McGill & McGill 1988 to the canon — banking to 45 comes
from the shape-parameter paper, not the 1984 ranking already cited, so
citing the latter for it would have misattributed the claim.

Also renames sin slugs to match their retitled pages, and rewrites the
summaries that had started echoing those titles.

Gives line-chart points their mark color explicitly; "point": true was
falling back to Vega's default blue on a red or green line.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ULE5RRxdQE1ebwefEd2eCM

src/charts/aspect-ratio-fixed.json

@@ -0,0 +1,36 @@
+{
+  "title": "Median Response Time (ms)",
+  "width": 330,
+  "height": 200,
+  "data": {
+    "values": [
+      { "month": "Jan", "ms": 182 },
+      { "month": "Feb", "ms": 190 },
+      { "month": "Mar", "ms": 196 },
+      { "month": "Apr", "ms": 205 },
+      { "month": "May", "ms": 212 },
+      { "month": "Jun", "ms": 219 },
+      { "month": "Jul", "ms": 228 },
+      { "month": "Aug", "ms": 234 },
+      { "month": "Sep", "ms": 241 },
+      { "month": "Oct", "ms": 250 },
+      { "month": "Nov", "ms": 259 },
+      { "month": "Dec", "ms": 268 }
+    ]
+  },
+  "mark": { "type": "line", "color": "#198038", "strokeWidth": 2, "point": { "color": "#198038" } },
+  "encoding": {
+    "x": {
+      "field": "month",
+      "type": "ordinal",
+      "sort": null,
+      "axis": { "labelAngle": 0, "title": null, "values": ["Jan", "Apr", "Jul", "Oct"] }
+    },
+    "y": {
+      "field": "ms",
+      "type": "quantitative",
+      "scale": { "domain": [170, 280], "zero": false },
+      "title": "ms"
+    }
+  }
+}