sam-ui-elements pipeline modernization + Angular 19→21 upgrade (SAM Angular upgrade — 4th tracer bullet) — IAEMOD-61187

Pipeline modernization **and** Angular version migration for `@gsa-sam/sam-ui-elements` — the **fourth tracer bullet** of the SAM design-library Angular upgrade epic (IAEMOD-57412), following sam-styles (#732 on GSA/sam-styles), ngx-uswds-icons (#26 on GSA/ngx-uswds-icons), and ngx-uswds (#184 on GSA/ngx-uswds). Coordination/tracking lives on the internal research repo (`mcaas-iae/iae-angular-upgrade`).

sam-ui-elements sits at **Angular 19** (published `@gsa-sam/sam-ui-elements@19.0.0`) and consumes only **sam-styles** (effectively done), so it is unblocked. It is the **only design library that is not a compiled Angular library**: it publishes **raw TypeScript source** (no `main`/`module`/`exports`; ~560 `.ts`), and the **consumer compiles it in-place**. Its formly is **bundled and load-bearing in prod**. See binding decision ADR-0009 on the coordination repo.

## What makes this repo different

- **Raw-source publish, not ng-packagr.** No library `ng build`; the consumer's own Angular compiler builds our source. Deep import paths (`/src/formly/**`, `/src/ui-kit/**`) are a **frozen contract** — no file moves this epic.
- **Branch of record is `ng-19`, not `master`.** `master` lags at Angular 18.0.2; `ng-19` is a strict content superset (the lone "master-only" commit is an empty merge bubble, 0 unique files). Reconcile by **merging `ng-19`→`master`** (no force).
- **No GitHub Actions** — only a commented-out CircleCI stub on Node 10.
- **Test harness is rotten.** Tests live in a separate `test-app/` that hard-links `../src` in, but the harness is **Angular 9 / TS 2.3 / ngx-formly 5** against a **library at Angular 19** — the 107 specs have not been compiling against a matching Angular. **Green-at-19 is the gate** before any migration.
- **formly kept + migrated, not deprecated.** `AbstractSamFormly` is imported in **15 files across 3 consumers, incl. prod** — unlike `uswds-formly` (separate package, 0 consumers, cleanly removed). So **ngx-formly 6→7 is in scope**, gating Angular 20.
- **rxjs already on 7** (peer `>=7.5.0`) — no rxjs migration; ~6 rxjs-6-only imports survive **in specs only**, fixed during the green baseline.

## Goal

Two bundled tracks, sequenced pipeline-first so the Angular migration lands on green CI:

1. **Pipeline/tooling** — inherit the pattern proven on sam-styles, icons, and ngx-uswds: registry-pinned `.npmrc`, normalized Node, ESLint (off tslint) with the a11y gate, Prettier, test + coverage, Dependabot, a PR template, dead-code pruning, Playwright smoke (replacing Protractor), and a tag-gated **live** `npm publish`.
2. **Angular migration** — stepped per-major chain 19 → 20 → 21 (LTS), one major at a time, with the **ngx-formly 6→7** migration as its own sub-issue gating the 20 step.

## Parallelism model

Serialize anything that mutates `package.json` / `angular.json` / `tsconfig`; parallelize disjoint file sets. Prettier and dead-code prune are **"stop the world"** (run solo). The Angular chain is a **serial train** — one major at a time, nothing else editing manifests/source in flight.

## Deferred (own follow-up issues, NOT this epic)

- Convert to a compiled ng-packagr library. **Prereq:** break the `src/ui-kit/filters → src/formly` (`FormlySAMUIModule`) dependency cycle.
- Replace the `test-app` copy-src-in harness with a standard in-repo Karma/Jest setup; retire `test-app`.

## Notes / scope boundaries

- **Migration is unblocked** — sam-ui-elements consumes only sam-styles. Only the **live publish** sub-issue carries the DevOps dependency (OIDC Trusted Publisher registration, ADR-0007).
- **Pin, don't break, the Angular-17 legacy consumer** (`iae-sam-front-end3`, IBM/legacy SAM). Major-locked peers make the version pin the natural outcome.
- **Consumer validation is mandatory before publish:** `npm pack --dry-run` + audit every frozen deep path against the three consumers.
- Jira epic (this repo): [IAEMOD-61187](https://gsa-standard.atlassian-us-gov-mod.net/browse/IAEMOD-61187)
- Program epic (SAM design-library Angular upgrade): IAEMOD-57412