Add grails-multi-module v8 guide (rename of grails-multi-project-build)

[jamesfredley]

Summary

Adds the Grails 8 Multi-Project Build guide as the sixth Grails 8 sample-app-flavour guide. Following PRs #479, #493, #494, #495, #496, #497.

A real-world layout: one shared-core Grails Plugin (Book domain + @Service(Book) BookService), two web apps (webapp-customer read-only catalog, webapp-admin full CRUD) that depend on the plugin via implementation project(':shared-core') and build to independent bootJars sharing the same data model.

Companion conf/guides.yml change (rename of upstream repo)

The upstream repo grails-guides/grails-multi-project-build was renamed to grails-guides/grails-multi-module. The v4 entry's sampleRef.repo (line 2923) is repointed at the new canonical name; the master and forensic branches are preserved on the renamed repo so the v4 guide still resolves.

Module layout

grails-multi-module/ |-- settings.gradle # include 'shared-core', 'webapp-customer', 'webapp-admin' |-- build.gradle # subprojects { repositories + useJUnitPlatform } |-- shared-core/ # Grails Plugin (web_plugin from forge) | -- grails-app/ | |-- domain/example/Book.groovy | -- services/example/BookService.groovy # @Service(Book) interface |-- webapp-customer/ # Grails web app | |-- build.gradle # implementation project(':shared-core') | -- grails-app/controllers/customer/CatalogController.groovy -- webapp-admin/ # Grails web app |-- build.gradle # implementation project(':shared-core') -- grails-app/controllers/admin/BookController.groovy

What's in the PR

• conf/guides.yml - new grails-multi-module registry entry (alphabetical insertion before grails-multi-project-build) plus the v4-entry repo-name update.
• guides/grails-multi-module/v8/guide/*.adoc - 12 chapters.
• guides/grails-multi-module/v8/snippets/ - vendored settings.gradle, build.gradle, Book.groovy, BookService.groovy, BookController.groovy (admin), CatalogController.groovy (customer).

Upstream sample app

grails-guides/grails-multi-module on the new grails8 branch (default). initial/ is intentionally empty and points readers at the three forge URLs. complete/ is the assembled three-module workspace.

Verification

• ./gradlew validateGuides -PvalidationMode=both returns 86 guides, 0 errors.
• ./gradlew renderGuide_grails_multi_module_8 --rerun-tasks --no-configuration-cache renders all 12 chapter HTML pages, no Unresolved directive errors.

Notes for reviewers

• This is the fifth Grails 8 PR that intentionally renames a grails-guides/* repo to a sharper name.
• The chapter on extracting a plugin from a monolith is a six-step migration recipe rather than a working code example - intentionally so, since the migration always starts from a different monolith.
• The chapter on failure modes documents three real production gotchas: circular plugin deps, GORM mapping conflicts when a domain class loads twice, and Liquibase coordination across two webapps.