perf(bundle): lazy-load the nunjucks template engine (#2535)

Closes #2531.

## Summary

The full `nunjucks` templating engine (~226KB) plus `ha-nunjucks`
(~46KB) — roughly **272KB, ~13% of the eager entry chunk** — was
statically imported and downloaded by every card on initial load, even
though templates only apply when a config value contains a `{{ … }}` /
`{% … %}` delimiter. Most cards use no templates and never need the
engine.

This defers the engine behind a dynamic `import('ha-nunjucks/dist')` so
it ships in a separate, on-demand chunk instead of the eager
`card-*.js`.

## Approach

The render path (`TemplateRenderer.renderRecursively`) is **kept
synchronous** — it is called from many synchronous hot paths
(condition/trigger evaluators, picture-elements rendering, actions,
folder matchers), and making it async would be a large, high-risk
refactor of the evaluation core.

Instead:

- **New `src/card-controller/templates/engine.ts`** — a module-level
singleton lazy loader (`loadTemplateEngine()` / `getTemplateEngine()`)
shared across all `TemplateRenderer` instances, plus a
`containsTemplate()` delimiter helper.
- **Delimiter gating** — strings without a delimiter never touch the
engine (the overwhelming majority of renders).
- **Pre-warm at config time** — because every template string originates
in the config, a new mandatory `TEMPLATE_ENGINE` initialization aspect
loads the engine before first render whenever the config contains a
delimiter. This **guarantees no raw `{{ … }}` flash**: content/condition
rendering is blocked until the engine is present for template-using
cards. Cards without templates never load it.

## Result

- nunjucks + ha-nunjucks move out of the eager chunk into a separate
chunk fetched only when a card actually uses templates.
- No change to the synchronous public render API; condition/trigger
evaluation core untouched.

## Tests

- New `engine.ts` loader coverage (concurrent load, cached reuse,
not-loaded fallback).
- The 11 existing test files that render real (delimiter-bearing)
templates declare their dependency explicitly via
`beforeAll(loadTemplateEngine)` — no global/implicit setup hook.
- Full suite green (4764 tests), lint and ts-prune clean, per-file 100%
coverage maintained for the affected directories.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_016ykWemdkZrgywvC71pHc6c

---
_Generated by [Claude
Code](https://claude.ai/code/session_016ykWemdkZrgywvC71pHc6c)_

tests/test-utils.ts

@@ -62,6 +62,7 @@ import type { PIPManager } from '../src/card-controller/pip-manager';
 import type { QueryStringManager } from '../src/card-controller/query-string-manager';
 import type { StatusBarItemManager } from '../src/card-controller/status-bar-item-manager';
 import type { StyleManager } from '../src/card-controller/style-manager';
+import type { TemplateManager } from '../src/card-controller/templates';
 import type { ViewItemManager } from '../src/card-controller/view/item-manager';
 import type { ViewManager } from '../src/card-controller/view/view-manager';
 import type { SubmenuInteraction, SubmenuItem } from '../src/components/submenu/types';
@@ -749,6 +750,14 @@ export const createLitElement = (): LitElement => {
   return element;
 };
 
+export const createMockTemplateRenderer = (): TemplateManager => {
+  const renderer = mock<TemplateManager>();
+  renderer.isLoaded.mockReturnValue(true);
+  renderer.renderRecursively.mockImplementation((_hass, value) => value);
+  renderer.renderRecursivelyAsType.mockImplementation((_hass, value) => value);
+  return renderer;
+};
+
 export const createCardAPI = (): CardController => {
   const api = mock<CardController>();
 
@@ -784,6 +793,7 @@ export const createCardAPI = (): CardController => {
   api.getQueryStringManager.mockReturnValue(mock<QueryStringManager>());
   api.getStatusBarItemManager.mockReturnValue(mock<StatusBarItemManager>());
   api.getStyleManager.mockReturnValue(mock<StyleManager>());
+  api.getTemplateManager.mockReturnValue(mock<TemplateManager>());
   api.getCameraTriggersManager.mockReturnValue(mock<CameraTriggersManager>());
   api.getViewItemManager.mockReturnValue(mock<ViewItemManager>());
   api.getViewManager.mockReturnValue(mock<ViewManager>());