overhaul: modular architecture, tests, doc generation, warning system, and UX improvements

- Modular block files: split into blocks-algo, blocks-data, blocks-proc
- New modules: sections.js (labelled canvas areas), proc-browser.js (procedure tree modal), lib-loader.js (lazy library loading), templates.js (project templates), errno-data.js (errno table)
- Test infrastructure: browser test suite (28 tests) and Node pure-function tests (75 tests)
- Warning system: unified .block-warn for disconnected inputs, unconnected error hooks, and zero-exit-in-error-path
- Error path highlighting: red flow paths and block borders for error-only subgraphs
- Input hooks: hidden by default, shown during linking; outputs always at bottom
- Block editor modal: pencil icon, colour picker for accent, compact field labels
- Doc generation: standalone node script extracts JSDoc -> docs/API.md (124 declarations)
- Persistence: save/restore sections, errno selection, block colours
- Fixes: _segmentsIntersect bug, WCEnd crash from closest(this), drawpath guard on detached elements

server.js

@@ -0,0 +1,42 @@
+const http = require("http");
+const fs = require("fs");
+const path = require("path");
+
+const PORT = 3000;
+const ROOT = __dirname;
+
+const MIME = {
+  ".html": "text/html",
+  ".js":   "application/javascript",
+  ".json": "application/json",
+  ".css":  "text/css",
+  ".svg":  "image/svg+xml",
+  ".png":  "image/png",
+  ".ico":  "image/x-icon",
+};
+
+const server = http.createServer((req, res) => {
+  let filePath = path.join(ROOT, req.url === "/" ? "index.html" : req.url);
+  filePath = path.normalize(filePath);
+
+  if (!filePath.startsWith(ROOT)) {
+    res.writeHead(403);
+    return res.end("Forbidden");
+  }
+
+  const ext = path.extname(filePath);
+  const contentType = MIME[ext] || "application/octet-stream";
+
+  fs.readFile(filePath, (err, data) => {
+    if (err) {
+      res.writeHead(404);
+      return res.end("Not found");
+    }
+    res.writeHead(200, { "Content-Type": contentType });
+    res.end(data);
+  });
+});
+
+server.listen(PORT, () => {
+  console.log(`Server running at http://localhost:${PORT}/`);
+});