host: stage lib/blogimport pickup — persist-backed blog content (Phase 4)

Staged cross-loop hand-off (not started here): when the cards-as-types work lands, swap
host/blog-lookup's in-memory registry for content/head over content:<id> streams
populated by lib/blogimport (merged to local architecture a746b6ab, 76/76). Adds a
Phase 4 checklist item + plans/blogimport-pickup.md with concrete steps (merge
architecture, apply blog-side published-posts draft, inject fetch_data as fetch-fn,
backfill, swap lookup, sync-verify parity gate).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plans/host-on-sx.md

@@ -185,6 +185,12 @@ lib/host/sxtp.sx                        subsystem APIs (feed/search/commerce/…
       welcome/` renders real HTML through Caddy. Needs Smalltalk+persist+content
       preloads + `(st-bootstrap-classes!)`+`(content/bootstrap!)` (self-bootstraps
       at load).
+- [ ] **persist-backed blog content via `lib/blogimport`** (STAGED, pick up after the
+      cards-as-types work). Swap `host/blog-lookup`'s in-memory registry for
+      `(content/head b post-id)` over `content:<id>` streams populated by `lib/blogimport`
+      (merged to local `architecture` `a746b6ab`, 76/76 — `git merge architecture` to
+      get it). Resolves Q-M4 (live source via injected `fetch-fn` = host `fetch_data`).
+      Full steps incl. the blog-side draft query + parity gate: `plans/blogimport-pickup.md`.
 - [ ] proxy-to-Quart fallback for un-migrated paths (strangler requirement before
       a real subdomain fronts users).
 - [ ] internal-HMAC middleware on `/internal/*` (service-to-service auth; protocol