docs(solutions): in-place header edit + APCu cache staleness (Part 3)

Document the trip publish-toggle cache-invalidation finding: an in-place
trip.md `published` edit under cache.check.method: folder + APCu driver stays
stale because the folder checksum is unchanged AND the web APCu store is
unreachable by a CLI clearcache — fixed with apcu_clear_cache() from the web
request. Cross-link the sibling grav-deleteall doc (the create/delete case) as
necessary-but-not-sufficient here, and add the Published/Draft trip status
concept to CONCEPTS.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mpdu3Dt1iVoozHwAMyjrbn

CONCEPTS.md

@@ -14,6 +14,11 @@ A single journey the blog is organised around — the top-level content entity.
 ### Active Trip
 The one Trip currently featured — set in site config and read by the home page and the post form. Switching the Active Trip is a deliberate, multi-file change; if the post form's target and the featured Trip fall out of sync, new posts land under the wrong Trip.
 
+### Published / Draft
+A Trip's visibility state. A **Published** Trip is listed publicly and reachable by anyone; a **Draft** Trip is hidden from anonymous visitors in the public trip list, while the signed-in owner still sees it (marked "Draft") and can flip it back. The owner toggles this per Trip from the trip list.
+
+Unpublishing the **Active Trip** additionally drops it from the public home page, which falls back to its between-trips landing. The toggle is owner-only; a Draft is a visibility control, not privacy — a Draft Trip's Entries, Stories, and media stay reachable by direct link.
+
 ### Entry
 A single dated journal post within a Trip — the atomic unit of the day-to-day travel log.
 *Avoid:* daily, journal post