fix(vite-plugin): hydrate islands exported by name

[romain-pm]

Closes #706 — part of EPIC #698 (developer experience).

The bug

// Form.client.tsx
export function Form() { … }        // named, not default

// default.server.tsx
<Island component={Form} />

Builds clean. Server-renders clean. In the browser the island requests /modules/<module>/undefined.js, the import fails, and the component never hydrates — for a form, the submit silently degrades to a native GET. The only clue is a console error.

insert-filename attaches the hydration metadata (__filename) to ExportDefaultDeclaration nodes only, so Component.__filename is undefined for a named export and <Island> interpolates it straight into the URL.

Change

Three coordinated pieces, all in this monorepo, released together:

• vite-plugin tags every export of a *.client.{jsx,tsx} file — export function, export const, export class, and the existing default — with __filename and the new __exportName. Declarations can't be wrapped in an expression, so named exports are tagged by a statement appended after them. Descriptors are configurable so a component exported twice (export const Foo = …; export default Foo;) is tagged twice instead of throwing.
• Island emits data-export (omitted for default exports, so nothing changes in existing markup) and now throws when a component carries no metadata at all, naming it and explaining which export forms are supported.
• the client loader reads data-export, falling back to default — so islands built with an older plugin keep working.

export { … } lists and re-exports remain unsupported: they cannot be tagged reliably, and they now land in the fail-fast path instead of producing undefined.js.

Verification

Fixtures (yarn workspace @jahia/vite-plugin test, 2 tests): a new named.client.tsx exporting two components by name is mounted alongside the existing default-export island; the test asserts the exact (__filename, __exportName) triples in the server bundle and that the client bundle really exposes both names. Snapshots regenerated.

E2E: the test module gains a named-export island, and the hydration spec gains a case asserting it hydrates and stays interactive, with a cy.intercept that fails the test if anything ever requests undefined.js.

Live on Jahia 8.2:

• named-export island renders with data-export="SampleNamedExportReact", hydrates, and increments on click (5 → 6);
• the two pre-existing default-export islands still hydrate (3/3 hydrated, no data-export on them);
• passing a component that was never declared in a client file now fails the render with <Island> received a component without hydration metadata: NotAClientComponent… instead of reaching the browser.

Note for the reviewer

Modules must be rebuilt with the matching plugin version to use named exports; already-deployed modules are unaffected (no data-export → loader falls back to default).

[github-actions]

🦜 Chachalog

javascript-modules minor

• <Island> now accepts client components exported by name, not only default exports: the Vite plugin tags every export of a *.client.{jsx,tsx} file, and the browser loader picks the right one. A component that carries no hydration metadata (declared outside a client file, or re-exported through an export { … } list) now fails at render with an explicit message, instead of silently making the browser request undefined.js. (Named exports in .client.tsx silently break island hydration (undefined.js) #706)

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