refactor: retire @action and get() from @ember/object

[josemarluedke]

Retires the two @ember/object APIs Frontile still uses, following two open Ember RFCs:

• RFC 1045 (Exploring) stops teaching @action and teaches arrow-function class fields instead. It does not deprecate @action.
• RFC 1234 (Proposed) deprecates EmberObject and its object model. Its table does not name the standalone get() function, but get() reads through that object model, so it is the likeliest next target.

Frontile uses nothing else on RFC 1234's list. Glimmer components define their own willDestroy / isDestroying / isDestroyed, which the RFC does not touch and @glimmer/component's docs still teach, so those are left alone. The same goes for guidFor and the Application.create() boot calls.

Commits

1. Replace get() with getPath (plus a follow-up commit from /simplify).
   • getConfigOption keys are flat, so it now uses plain property access.
   • Field#fieldValue needs dotted paths, so it uses a new getPath in utils/nested-data.ts. That puts it next to flattenData / unflattenData, and it is exported with them. It is a small public API addition.
   • It reads accessors, so @tracked fields and getters on class-instance formData work as they did with get().
   • It refuses __proto__ / constructor / prototype segments, including an own __proto__ key like the one JSON.parse produces. Field names can come from untrusted schemas, and Ember's get() would follow those segments to Object.prototype.
   • The tests were written first, and each guard was checked by removing it and watching the tests fail.
2. @action → arrow fields in packages/*/src, including the co-located component .md files. Docfy renders those as live demos, and they are what consumers copy.
3. The same in docs/, site/app, and test-app.

Review notes

• One exception. site/app/routes/application.ts didTransition keeps @action. It is a router event, looked up in the route's actions hash, and an arrow field is not in that hash.
• Mechanical conversion. The conversion was scripted. Aside from the removed decorators and imports, it only rewrites method headers (name = [async] (params): Ret => {) and turns closing } into };. Added and removed lines match one for one.
• Initialization order. Class fields initialize in declaration order, so a field initializer that reads an action declared below it would get undefined. I checked every this.<action> that appears above its declaration, and all of them are inside methods, callbacks, or templates.
• Overrides. Nothing calls an action through super. changeset-form's validate overrides still work because the base class and its subclasses are all fields.

Verification

• pnpm build
• test-app: 2202 tests, 0 failures (3 skipped)
• site suite: 26/26
• lint:types is clean for frontile, changeset-form, and forms-legacy.
• eslint is clean on the changed files.
• The template-lint errors in field.gts / form.gts / native-select.gts and the site/test-app type errors all exist on main and are on lines this PR does not change.

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