ci(server): strict type errors per file may only shrink

tsconfig.strict.json is the build config with strictNullChecks and
noImplicitAny on. lint:strict runs tsc over it and holds the errors per file
to scripts/strict-baseline.json (204 in 109 files today): a file may only
lose errors, a new file starts at none, and an entry above its count fails
until --update lowers it. An error outside any file or an unreadable tsc run
fails closed. It runs in the server job; the build itself stays on
strict: false.

server/scripts/lib/tsc-errors.mjs

@@ -0,0 +1,29 @@
+/*
+ * Reading tsc's --pretty false output for lint:strict (scripts/strict-lint.mjs).
+ * Kept free of imports so the unit tests can load it on their own.
+ */
+
+/** `src/a.ts(3,7): error TS2322: ...` as tsc --pretty false prints it. */
+const FILE_ERROR = /^(.+?)\((\d+),(\d+)\): error (TS\d+): /;
+/** An error tied to no file, such as a config problem. */
+const GLOBAL_ERROR = /^error (TS\d+): /;
+
+/**
+ * The errors per file in tsc's output, keyed by the path from the server root
+ * with forward slashes, and the lines of every error that names no file.
+ * Continuation lines (indented) belong to the error above and do not count.
+ */
+export function countErrors(output) {
+  const counts = {};
+  const global = [];
+  for (const line of output.replace(/\r\n?/g, '\n').split('\n')) {
+    const match = FILE_ERROR.exec(line);
+    if (match) {
+      const key = match[1].split('\\').join('/').replace(/^\.\//, '');
+      counts[key] = (counts[key] ?? 0) + 1;
+    } else if (GLOBAL_ERROR.test(line)) {
+      global.push(line);
+    }
+  }
+  return { counts, global };
+}