EPBDS-16211 React 19: declare it, and gate it like 16 and 17

[AlexSamBY]

On master since #130 merged, which fixed the npm install peer conflict this change needed gone to add a devDependency. The rebased tree is identical to the one the local gates below ran on.

The flip

peerDependencies: react and react-dom now read ^16.14.0 || ^17.0.0 || ^18.0.0 || ^19.0.0. The range is widened additively, so hosts on 16.14, 17 and 18 change nothing. This closes §8's fast path:

• gates 1 (Semantic UI React gone) and 3 (defaultProps, then the automatic JSX runtime in EPBDS-16211 G: compile JSX with the automatic runtime #129) were met;
• gate 2 (the form stack) lapsed when dependencies was emptied (EPBDS-16211 H9: burn the dev-tooling audit down to zero, and record today's decisions #128 records why);
• this PR is gate 4.

How 19 is gated, in three matrices

	16.14	17	18	19
whole jest suite	test:react16	test:react17	npx jest / test:coverage	test:react19 (new)
packed artifact server-rendered	test:pack:peers	test:pack:peers	test:pack:consumer	test:pack:peers (added 19.3.0)
published .d.ts compiled	test:types	test:types	test:types	test:types (added @types/react 19.3.0)

• The suite on React 19 is a fixture leg, like 16.14 and 17. scripts/fixtures/react19 is an install-only package pinning react/react-dom 19.3.0. Plain npm ci nests it, it is mapped in at resolve time, and npm run test:react19 runs it locally too. The harness still asserts the loaded React (19.3.0) and scheduler (0.28) in every worker.
• What differs for 19. React 19 has no legacy root and ships react-dom/client, so the leg renders through createRoot as the default suite does. floors.js records legacyRoot: false, which skips three things that exist only for 16/17: the RTL legacyRoot patch, the ReactDOM.render assertion and the react-dom/client stub.
• CI. The new react-19 job replaces the continue-on-error react-19-advisory job, which installed React 19 over the lock with --legacy-peer-deps; the new job needs neither. No required checks are configured on master today, so react-19 gates exactly as react-16-floor and react-17 do. If branch protection ever named react-19-advisory, it should name react-19 now.
• Types. The published declarations compile against @types/react 19, with both the interop default import and the direct CommonJS import. That matters because 19 removed the global JSX namespace. gen-ts stays on the dev tree's 18; the plan's "pin @types/react@19 for gen-ts" item was written assuming the dev tree would move to 19, and it does not.

Docs

• README and the demo's docs page state the new range.
• The demo's changelog (Unreleased) says React 19 is supported and how it is tested. It also records EPBDS-16211 G: compile JSX with the automatic runtime #129's change: the bundle imports react/jsx-runtime from the host's React, which a host that aliases react to one copy has to alias too. Two older Unreleased paragraphs that said the range was not widened yet are updated, so the section does not contradict itself.
• CLAUDE.md lists the three React legs.
• The plan closes §8's gate 4, the CI-coverage row, the gen-ts row, the Appendix C "React 19 flip" checklist and §10's Phase 7 React 19 item.

Gates

All exited 0:

• lint:js, lint:css, typecheck, typecheck:contract;
• css:fixture:check, docs:props:check, docs:views:check;
• test:env-flags, test:types, test:coverage, build;
• test:pack, test:pack:peers (16.14.0, 17.0.2, 19.3.0);
• test:react16, test:react17, test:react19, 2907 tests on each, as on 18;
• test:e2e, 41 Playwright tests;
• the manifest contract.

Two first runs lost a jest worker to SIGSEGV, each killing one suite before its tests ran: test:react19 once and test:react17 once. That is the Node 24 V8 garbage-collector crash (nodejs/node#62393), fixed on v24.x-staging but not in a release yet. Both passed 2907/2907 on rerun. CI runs Linux, where it has not been seen.

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