refactor: model bin errors as neverthrow Result values (task 0003)

Split the view command out of the entry point and make its failure a
value rather than an exception. index.ts is now scaffolding plus
dispatch; view.ts owns the behaviour, returning Result<string, ViewError>
(a plain-data tagged union) via neverthrow, with try/catch confined to a
readFile adapter. The entry point matches the Result at the boundary.

Because viewFile is a pure in-process function it is now unit-tested
directly (and counted by coverage), while the subprocess tests stay as
the end-to-end check. ADR 0008 records the errors-as-values convention.

packages/bin/src/view.ts

@@ -0,0 +1,27 @@
+import { readFileSync } from "node:fs";
+import { parse } from "@kitchen-md/core";
+import { err, ok, type Result } from "neverthrow";
+import { render } from "./render.ts";
+
+export type ViewError = { tag: "read-failed"; path: string; cause: string };
+
+export function viewFile(path: string): Result<string, ViewError> {
+  return readFile(path).map((content) => render(parse(content)));
+}
+
+export function formatViewError(error: ViewError): string {
+  switch (error.tag) {
+    case "read-failed":
+      return `cannot read '${error.path}': ${error.cause}`;
+  }
+}
+
+// The one place a throwing API is turned into a Result; nothing above this leaks exceptions.
+function readFile(path: string): Result<string, ViewError> {
+  try {
+    return ok(readFileSync(path, "utf8"));
+  } catch (error) {
+    const cause = error instanceof Error ? error.message : String(error);
+    return err({ tag: "read-failed", path, cause });
+  }
+}