feat: optional keys-for-cache (KFC) support

[erikfrerejean]

What & why

Adds optional out-of-the-box support for silverstripe-terraformers/keys-for-cache (KFC) so grid elements get reliable, auto-invalidating cache keys when the module is installed — without making it a hard dependency. It is added under suggest, and all wiring is guarded by Only: moduleexists.

Approach

The integration is config-only — no PHP, no imports, no PHPStan stubs. KFC already applies its CacheKeyExtension to DataObject globally, so _config/keys-for-cache.yml only:

• sets has_cache_key: true on GridElement (inherited by Section/Row/Column/ContentElement and any consumer subclass);
• declares cares downward through the has_many relations (Section→Rows, Row→Columns, Column→Elements), deliberately avoiding the polymorphic Parent edge. A change to any descendant transitively invalidates its ancestors' keys.

Consumers cache per-section in their own templates: <% loop $Sections %><% cached $CacheKey %>$Me<% end_cached %><% end_loop %>. We do not opt SiteTree into has_cache_key (only classes we own).

Test environment consolidation

The previous per-module app-fluent Docker service is replaced by a single app-modules env that installs all optional modules (Fluent and KFC). Base app stays the zero-optional-deps baseline; app-modules is the everything-optional env. This touches compose.yml, Makefile (make test-modules), composer.modules.json, phpunit.xml.dist (new kfc suite), and CI. Trade-off: the kfc suite runs with Fluent also present (the realistic combined target); the base env still proves "works with nothing optional".

KFC's SS6-compatible release is ^3.0 (1.x/2.x cap at framework ^5).

Not included — owned-media caching (deferred)

Caching media-image edits was dropped: KFC cannot cares about a has_one whose target has no reciprocal relation, and MediaImage/VideoCustomThumbnail point to the shared Image class. Editing media via an element's CMS form re-saves the element (which does invalidate via the downward cares); only editing an in-use Image directly in asset-admin is uncovered. Documented as a known limitation; a per-element owned media model that KFC can traverse is a planned separate project.

Tests

New tests/Integration/KeysForCache/ suite guards: cache-key presence, transitive ancestor invalidation, cross-tree isolation, and draft/live separation. (Invalidation tests use KFC's documented ProcessedUpdatesService::flush() to defeat its per-request dedup; assertions are not weakened.)

Verification (local):

• Base env, KFC absent: OK (1199 tests, 3919 assertions)
• Optional-modules env (integration,functional,fluent,kfc): OK (766 tests, 2444 assertions)

Docs

• docs/architecture/keys-for-cache.md — what activates, the cares graph, the <% cached %> recipe, the Fluent global_key note, the media limitation, and the per-write cost note.
• New gotcha in CLAUDE.md (via APM sources): editing a single-file bind mount (composer.json) while dev containers run breaks it inside the container until --force-recreate.