Surface companion plugins on downloads page when target Grails major has not yet shipped

[jamesfredley]

Summary

• Adds a new Plugins for upcoming Apache Grails releases section to /download.html that lists Apache-released companion plugins whose target Grails major has no entry yet in coreReleases: (e.g. the existing companionArtifacts['8'] -> grails-publish 1.0.0-M1 block).
• Introduces SiteMap.orphanCompanionMajors(File) to compute those orphan majors (descending, empty companion lists skipped).
• Adds DownloadPage.renderCompanionsOnlyCard(major, companions) - a card with the same guide-group / release-grid visual vocabulary as the existing sections, minus the core Source / Binary / Wrapper / Grails Release Notes block (since that core release does not exist yet).

Why

download.html iterates Grails majors via activeMinorLines() and latestPreReleasePerMajor(), both of which read only coreReleases:. That meant a companion plugin published ahead of its target core (e.g. grails-publish 1.0.0-M1 for the upcoming Grails 8) was silently invisible on the downloads page until Grails 8 shipped its first milestone. This PR closes that gap.

Auto-convergence (no future YAML edits)

The moment any release for an orphan major (stable, milestone, or RC) lands in coreReleases::

• orphanCompanionMajors() drops the major.
• The new section disappears (or shrinks to other orphans).
• The existing per-major card in Current Releases or Pre-release (Apache-released) automatically picks up the same companions through companionArtifactsFor().

No manual YAML migration needed at release time.

Section ordering

```
Current Releases
Pre-release (Apache-released)
Plugins for upcoming Apache Grails releases <- NEW
Older Versions
Get Started
```

Test coverage

6 new `SiteMapSpec` cases mirror the existing `companionArtifactsFor` tests (`@TempDir` + `releasesFile()` helper):

1. empty when no `companionArtifacts:` block exists
2. empty when every companion major has a stable core release
3. empty when the companion major has only a pre-release in `coreReleases:` (existing pre-release card handles it)
4. returns the major when a companion exists but no core release of any kind exists
5. returns multiple majors in descending order
6. skips majors whose companion list is empty

All 6 pass. Pre-existing `ValidateGuidesTaskSpec > shape mode rejects a non-40-char SHA` failure exists on `master` independently of this change.

Verification

• `./gradlew :buildSrc:test` -> all SiteMapSpec tests green.
• `./gradlew build` -> succeeds.
• Inspected `build/dist/download.html` rendered output: new section present in correct position, card titled Apache Grails 8 Plugins, links resolve to `apache-grails-publish-1.0.0-M1-src.zip` plus `.sha512` / `.asc` on `downloads.apache.org`, release notes -> `apache/grails-gradle-publish/releases/tag/v1.0.0-M1`.

Scope notes

• Download page only. `DocumentationPage` does not currently render companion artifacts at all, so applying the same orphan logic there would be a new responsibility for that page; out of scope here.
• No `releases.yml` schema change. The orphan-vs-attached distinction is inferred from existing data; nothing in the YAML moves when Grails 8 ships.