refactor(core,cli,engine): apply /simplify findings on getVariables PR

- core/runtime/getVariables.ts: collapse the noisy three-step type-guard
  re-cast into a single `Record<string, unknown>` narrow with early-continue
  guards. Same behaviour, ~6 lines shorter.
- cli/commands/render.ts: separate VariablesParseError from UI strings.
  parseVariablesArg now returns a kind-discriminated error
  (`conflict | read-error | parse-error | shape-error`) and the wrapper
  resolveVariablesArg owns the title/message mapping via
  `variablesErrorMessage`. Keeps the parser pure of presentation strings.
- cli/commands/render.test.ts: lift the `await import("./render.js")` into
  a `beforeAll`, add a typed `expectErr` helper, assert on the structured
  error kind instead of message-string regexes. Same coverage, less noise.
- engine/services/frameCapture.ts: replace the `as unknown as { ... }`
  double-cast with a single named `WindowWithVariables` alias inside the
  page closure.

All affected suites green (core getVariables 9, cli render 12, cli
dockerRunArgs 13).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/runtime/getVariables.ts

@@ -38,14 +38,10 @@ function readDeclaredDefaults(root: Element | null): Record<string, unknown> {
 
   const out: Record<string, unknown> = {};
   for (const entry of parsed) {
-    if (
-      entry &&
-      typeof entry === "object" &&
-      typeof (entry as { id?: unknown }).id === "string" &&
-      "default" in entry
-    ) {
-      out[(entry as { id: string }).id] = (entry as { default: unknown }).default;
-    }
+    if (!entry || typeof entry !== "object") continue;
+    const e = entry as Record<string, unknown>;
+    if (typeof e.id !== "string" || !("default" in e)) continue;
+    out[e.id] = e.default;
   }
   return out;
 }