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

jest.config.js

@@ -7,6 +7,10 @@ export default {
   moduleNameMapper: {
     // Resolve .js imports to .ts source files (ESM convention in TS source)
     '^(.+)\\.js$': ['$1.ts', '$1.tsx', '$1.js'],
+    // github-slugger v2 is ESM-only; jest's CJS runtime cannot require it.
+    '^github-slugger$': '<rootDir>/__mocks__/github-slugger.js',
+    // Same for the browser ESM build of csv-parse — map to the CJS build.
+    '^csv-parse/browser/esm/sync$': 'csv-parse/sync',
   },
   transform: {
     '^.+\\.tsx?$': [