feat(cli): Add tttk check validator

Statically validate a directory of tttk documents and report structured diagnostics with file/line positions, exit codes, and --json output. Checks cover code-block roles, unresolved content references, tag blocks, element names, links, anchors, and assets. Runs over the same content registry as the app.

src/cli/validate/checks/context.ts

@@ -0,0 +1,39 @@
+/**
+ * Shared context passed to every per-document check.
+ */
+
+import { posix } from "path";
+import type { ContentRegistry } from "../../content-registry.js";
+import type { DiagnosticSink } from "../diagnostics.js";
+import type { ScanResult } from "../scan.js";
+import type { SourceFile } from "../source.js";
+
+export interface CheckContext {
+  source: SourceFile;
+  scan: ScanResult;
+  /** Display path: relative to the root, posix, no leading slash. */
+  file: string;
+  /** Registry key: posix with a leading slash. */
+  pathKey: string;
+  registry: ContentRegistry;
+  /** Raw contents of every indexable file, keyed like the registry. */
+  index: Record<string, string>;
+  /** Absolute root directory of the check. */
+  rootDir: string;
+  sink: DiagnosticSink;
+}
+
+export type Check = (ctx: CheckContext) => void;
+
+/** Strip the registry key's leading slash for display. */
+export function displayPath(key: string): string {
+  return key.replace(/^\//, "");
+}
+
+/** Resolve a reference written in `pathKey` to a registry key. */
+export function resolveKey(pathKey: string, ref: string): string {
+  if (ref.startsWith("/")) return posix.normalize(ref);
+  const dir = posix.dirname(pathKey);
+  const joined = posix.normalize(posix.join(dir, ref));
+  return joined.startsWith("/") ? joined : `/${joined}`;
+}