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/markdown/index.ts

@@ -2,7 +2,6 @@ import { Marked, type MarkedExtension } from "marked";
 import { createDirectives, presetDirectiveConfigs } from "marked-directive";
 import markedAlert from "marked-alert";
 import markedMermaid from "./mermaid";
-import markedTable from "./table";
 import { gfmHeadingId } from "marked-gfm-heading-id";
 import markedColumns from "./columns";
 import markedCodeBlockYamlTag from "./code-block-yaml-tag";
@@ -14,7 +13,6 @@ const marked = new Marked()
   .use(gfmHeadingId())
   .use(markedAlert())
   .use(markedMermaid())
-  .use(markedTable())
   .use(markedCodeBlockYamlTag())
   .use(
     createDirectives([