… binding but keeps its property reads) (#10109)

## What

Adds a Rust integration-test fixture at `crates/rolldown/tests/rolldown/issues/10099/` closes #10099.

## Why

#10099: importing only `dayjs` from `element-plus` emitted a bundle that threw `ReferenceError: defaults_default is not defined`. The `sideEffects:false` barrel (`index.mjs`) re-exports an **external** module's default (`dayjs`) as a passthrough while reading `defaults_default.install`/`.version` at the top level. Importing only that passthrough force-includes the barrel; under `experimental.lazyBarrel` the tree-shaker dropped the `defaults_default` import **and** its definition but kept the property reads → dangling reference.

Fixed by #10080 (gate `sideEffects:false` modules' side effects on body demand). This PR adds the regression test for the specific element-plus shape, which none of #10080's fixtures cover: here the inclusion driver is an **external-default passthrough** (the sibling fixtures use local passthroughs or dynamic-import wrapping).

## The fixture

- `_config.json`: `external:["extdep"]`, `experimental.lazyBarrel:true`, `treeshake.moduleSideEffects:false`
- barrel `index.js` default-imports `defaults.js`, reads `.install`/`.version`, and re-exports the external default as `extdep`
- entry `main.js` imports only `{ extdep }` and asserts it resolves
- `node_modules/extdep/` stub so the external passthrough resolves at runtime

`experimental.lazyBarrel: true` is load-bearing: the bug is lazyBarrel-gated and lazyBarrel is default-off since #10071, so without it the test would pass on both the base and the fix.

## Verification

- **Green** on current `main` (fix present): the harness executes the compiled entry (`expectExecuted` default) and the snapshot is clean — no `defaults_default`.
- **Red** on the pre-fix base: the exact fixture source + config through published `rolldown@1.1.3` throws `ReferenceError: defaults_default is not defined`.

Test-only; no source changes.