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)_

src/config/schema/actions/custom/generated-action.ts

@@ -0,0 +1,31 @@
+import { z } from 'zod';
+
+import type { CardActionsAPI } from '../../../../card-controller/types';
+import type { TriggerData } from '../../../../condition-trigger/triggers/types';
+import type { ActionConfig } from '../types';
+import { advancedCameraCardCustomActionsBaseSchema } from './base';
+
+interface GeneratedActionContext {
+  api: CardActionsAPI;
+  triggerData?: TriggerData;
+}
+
+// Returns the action(s) to run, or null to generate nothing.
+export type ActionGenerator = (
+  context: GeneratedActionContext,
+) => ActionConfig | ActionConfig[] | null;
+
+// An internal action (not user-configurable) that, when executed, generates and
+// runs a concrete action
+export const GENERATED_ACTION = '__GENERATED_ACTION__';
+export const generatedActionConfigSchema =
+  advancedCameraCardCustomActionsBaseSchema.extend({
+    advanced_camera_card_action: z.literal(GENERATED_ACTION),
+
+    // Validated with z.custom rather than z.function (which internal_callback's
+    // callback uses) because the return type is an action config: typing it via
+    // z.function would require importing actionConfigSchema from the module
+    // that imports this one, a circular import.
+    generator: z.custom<ActionGenerator>((value) => typeof value === 'function'),
+  });
+export type GeneratedActionConfig = z.infer<typeof generatedActionConfigSchema>;

src/config/schema/actions/types.ts

@@ -14,6 +14,7 @@ import { cameraSelectActionConfigSchema } from './custom/camera-select';
 import { viewDisplayModeActionConfigSchema } from './custom/display-mode';
 import { effectActionConfigSchema } from './custom/effect';
 import { generalActionConfigSchema } from './custom/general';
+import { generatedActionConfigSchema } from './custom/generated-action';
 import { internalCallbackActionConfigSchema } from './custom/internal';
 import { logActionConfigSchema } from './custom/log';
 import { mediaPlayerActionConfigSchema } from './custom/media-player';
@@ -79,6 +80,7 @@ const advancedCameraCardCustomActionSchema = z.union([
   cameraSelectActionConfigSchema,
   effectActionConfigSchema,
   generalActionConfigSchema,
+  generatedActionConfigSchema,
   internalCallbackActionConfigSchema,
   logActionConfigSchema,
   mediaPlayerActionConfigSchema,