…main (#10318)

## Problem

Since #10164 (and #10293 which unified the checkouts), `packages/vite-tests/run.ts` ran Vite's test suite on the commit pinned by the vite submodule gitlink. The `rolldown-canary` branch of vitejs/vite is regularly rebased and force-pushed, so any pinned commit rots quickly: the current pin `4d1480ca` is no longer on any branch. Worse, test adjustments landing on `rolldown-canary` cannot reach rolldown CI without a manual pin bump. That is exactly why CI on `main` is red right now: the `js-sourcemap` inline snapshot was updated on `rolldown-canary` on 2026-07-15 (for the output change from #10249), but the pin predates that update.

## Change

Restore the pre-#10164 approach from #7633. `run.ts` now does:

```
git clone --branch rolldown-canary https://github.com/vitejs/vite.git
git rebase origin/main
```

so test adjustments landing on `rolldown-canary` take effect right away, and new tests from Vite `main` surface incompatibilities with rolldown early. The rebase identity comes from the existing "Configure Git" step in the vite-test CI jobs.

All inline spec patches are removed from `run.ts`. Test adjustments belong on the `rolldown-canary` branch itself, not in a patch layer inside this repo:

- The `css-codesplit` style-/style2- patch was already dead code: upstream Vite `main` has had the swapped assertions since vitejs/vite#22922.
- The `assets` raw-query skip (#8839) is obsolete: the test passes on current rolldown `main`, in both serve and build.
- The `hmr-full-bundle-mode` invalidate patch is removed. This one still needs a spec fix on `rolldown-canary`: with client-side HMR there is no "hmr invalidate" server log anymore, so the spec should assert that `.invalidation-parent` becomes `child updated` instead. Until that lands on the canary branch, `test-serve` fails this single test.

Also reverts the `tsconfig.json` include added for the now-removed `checkout.ts` import, and updates the `repo-structure.md` description.

## Verification

Full local run against current rolldown `main` (debug build):

| Suite | Result |
| --- | --- |
| test-unit | 65 files passed |
| test-serve | 1 real failure: `hmr-full-bundle-mode > invalidate` (expected, see above) |
| test-build | 93 files passed, `js-sourcemap` snapshot green again |

`environment-react-ssr > deps reload` failed once under full-suite load but passes in isolation (timing flake with the slow debug binding, not a regression).

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