e2e: scheduled rotation tests a stale daily because /rstudio/latest/index.json lags the branch rollover

## Summary

The scheduled e2e rotation runs the tests from `main` against a daily build from the
`rel-yellow-yarrow` (2026.08) branch. Any test that asserts behavior added to `main`
after the 2026.08 branch cut fails on every platform, because the binary under test
does not contain the code.

This is not a test defect and not flake. The tests are correct and the product is
correct. Only the pairing is wrong.

## Symptom

Scheduled runs [31303035336](https://github.com/rstudio/rstudio/actions/runs/31303035336)
(2026-08-09) and [31371013310](https://github.com/rstudio/rstudio/actions/runs/31371013310)
(2026-08-10) failed the same tests on Ubuntu 24, Rocky 9, Rocky 10, Ubuntu 26,
macOS 14, macOS 15, macOS 26, and Windows Server 2025:

- `tests/panes/layout/panes.test.ts` -- 8 of the 9 tests added by #18454
- `tests/panes/editor/cpp_chunk_indent.test.ts` -- the test added by #18470

A representative failure:

```
panes/layout/panes.test.ts:760 › Restoring a zoomed pane from its header button ends the zoom (#18444)

Error: zoom bookkeeping should be cleared after escaping the zoom
Expected: false
Received: true
```

That is exactly the pre-#18454 behavior. The pane header button restored the vertical
split through `DualWindowLayoutPanel` but left `PaneManager.maximizedTab_` set.
`hookPaneMaximize` (`src/gwt/src/org/rstudio/studio/client/workbench/ui/PaneManager.java:1344`)
is the code that routes the gesture through `restoreLayout()`, and it is absent from
the build under test.

The failure set confirms the diagnosis. #18454 added 9 tests, 8 fail, and the only
one that passes -- `'The pane header button still maximizes when nothing is zoomed'`
-- is the one that asserts behavior which predates the fix.

## Root cause

The rotation installs the build named by `https://dailies.rstudio.com/rstudio/latest/index.json`.
That JSON object is stale. The HTML page at the same path is not:

| URL | `last-modified` | Serves |
| --- | --- | --- |
| `/rstudio/latest/` (page) | Mon, 10 Aug 2026 19:36 GMT | Autumn Hawkbit, `2026.09.0-daily+11` |
| `/rstudio/latest/index.json` | **Sat, 08 Aug 2026 05:33 GMT** | Yellow Yarrow, `2026.08.0-daily+186` |
| `/rstudio/autumn-hawkbit/index.json` | Mon, 10 Aug 2026 18:32 GMT | Autumn Hawkbit, `2026.09.0-daily+11` |

So the `latest` alias rolled over to `main` for browsers but not for the JSON manifest
that CI consumes. The per-branch manifest is current and correct.

Commit check:

```
build installed by the rotation: 2026.08.0-daily+186 -> 0731c0d19d / a249c8fa0c (rel-yellow-yarrow)
build the page advertises: 2026.09.0-daily+11 -> 2f648f3a0d (main)

8b3b385e55 (#18454, pane zoom) NOT in a249c8fa0c IS in 2f648f3a0d
7ed4e560a0 (#18470, cpp chunk indent) NOT in a249c8fa0c IS in 2f648f3a0d
```

Reproduce:

```bash
curl -sSI https://dailies.rstudio.com/rstudio/latest/index.json | grep -i last-modified
curl -fsSL https://dailies.rstudio.com/rstudio/latest/index.json | jq -r '.branch, .version'
curl -fsSL https://dailies.rstudio.com/rstudio/autumn-hawkbit/index.json | jq -r '.branch, .version'
```

## Why PR runs are unaffected

`os-test-e2e-rstudio-pr.yml` already guards against this. Its `decide()` function
requires the daily's commit to contain the PR's merge base, and falls back to a source
build when it does not. #18454 and #18470 both touched `src/`, so both built from
source and went green. The guard fails closed, which is why the problem only shows up
in the rotation.

`os-test-e2e-rstudio-scheduled.yml` has no equivalent check. It installs whatever
`latest/index.json` names and runs `main`'s tests against it.

## Blast radius

Nine workflows read the stale manifest:

```
.github/workflows/os-test-e2e-rstudio-desktop-linux.yml
.github/workflows/os-test-e2e-rstudio-desktop-os-autocomplete-scheduled-windows-server-2025.yml
.github/workflows/os-test-e2e-rstudio-desktop-os-macos-14-arm64.yml
.github/workflows/os-test-e2e-rstudio-desktop-os-macos-15-x86_64.yml
.github/workflows/os-test-e2e-rstudio-desktop-os-macos-26-arm64.yml
.github/workflows/os-test-e2e-rstudio-desktop-os-windows-server-2025.yml
.github/workflows/os-test-e2e-rstudio-pr.yml
.github/workflows/os-test-e2e-rstudio-scheduled.yml
.github/workflows/os-test-e2e-rstudio-server-os-ubuntu-24.yml
```

The Windows one uses PowerShell (`Invoke-RestMethod`); the rest use `curl` + `jq`.

## Potential fixes

### 1. Resolve the manifest per branch (recommended)

Derive the manifest path from `version/RELEASE`, which is already in the workspace by
the time the installer is resolved. Lowercase it and replace the space with a hyphen:
`Autumn Hawkbit` -> `autumn-hawkbit`, `Yellow Yarrow` -> `yellow-yarrow`. Both paths
exist and both are current.

```bash
BRANCH_SLUG=$(tr 'A-Z ' 'a-z-' < version/RELEASE)
MANIFEST_URL="https://dailies.rstudio.com/rstudio/${BRANCH_SLUG}/index.json"
if ! MANIFEST=$(curl -fsSL --retry 3 --retry-delay 5 "$MANIFEST_URL"); then
echo "::error::No daily manifest for branch '$BRANCH_SLUG' at $MANIFEST_URL."
exit 1
fi
```

This stops depending on the `latest` alias, so a stale alias cannot mis-pair the build
again. It also pairs a release-branch PR with that branch's daily, and it removes the
needless source builds the PR workflow does today whenever `latest` lags.

Cost: nine files, one of them PowerShell. Worth factoring the resolution into a single
composite action in the same change.

### 2. Add the merge-base staleness guard to the rotation

Port `decide()` from `os-test-e2e-rstudio-pr.yml` into
`os-test-e2e-rstudio-scheduled.yml`, so a daily that does not contain `main`'s HEAD
triggers a source build.

Correct, and it makes the rotation self-defending against any future skew. But while
the manifest is stale it would build every engine from source -- more than ten of
them -- which is a large cost for a rotation whose stated purpose is to test the
shipped daily.

Best used together with option 1, as a backstop rather than the primary fix.

### 3. Fix the publisher

`/rstudio/latest/index.json` should be regenerated whenever `/rstudio/latest/` is.
This needs whoever owns the dailies site, so it is not fixable in this repo. Even
after it is fixed, option 1 is still worth doing: a consumer that names its own branch
cannot be mis-pointed by an alias.

### Not recommended: skip or version-gate the tests

Adding `test.skip` or a version guard to the affected specs would turn a build-pairing
problem into permanently disabled coverage, and would hide the next occurrence.

## Suggested order

1. Report the stale `index.json` to the dailies owners (option 3).
2. Land option 1 so CI stops depending on `latest`.
3. Land option 2 as a backstop.

Start with .github/workflows/os-test-e2e-rstudio-scheduled.yml and compare its daily resolution with decide() in os-test-e2e-rstudio-pr.yml. Review the other listed workflow files, including the PowerShell Windows workflow, and determine how the branch-specific manifest should be resolved or factored into a composite action. Done means scheduled workflows no longer depend on the stale latest manifest and continue to install the matching daily.

Written by the indexing model from the issue text.