refactor: remove implicit content sniffing and table conversion

- Split loadCSV into parseCSVString (content) and loadCSVFromPath
  (path); drop isCSV/looksLikeCsv heuristics
- Delete coerceSparkTables and markedTable label-header magic;
  plain markdown tables now render as plain tables
- Add explicit markdown role=spark-table fence syntax that converts
  pipe tables to CSV at scan time, with dice-header validation
- Map ESM-only github-slugger and csv-parse browser build to CJS in
  jest config; add content-registry tests

src/components/md-deck/hooks/deckStore.ts

@@ -1,7 +1,7 @@
 import { createStore } from "solid-js/store";
 import yaml from "js-yaml";
 import { calculateDimensions } from "./dimensions";
-import { loadCSV, CSV } from "../../utils/csv-loader";
+import { loadCSVFromPath, CSV } from "../../utils/csv-loader";
 import { formatLayers } from "./layer-parser";
 import * as layerCrud from "./layer-crud";
 import type {
@@ -461,7 +461,7 @@ export function createDeckStore(initialSrc: string = ""): DeckStore {
     setState({ isLoading: true, error: null, src: path, rawSrc: rawSrc });
 
     try {
-      const data = await loadCSV(path);
+      const data = await loadCSVFromPath(path);
 
       if (data.length === 0) {
         setState({