host: hand off the native SX-island editor (browser-capable session)

The editor is the interactivity layer — it belongs on the --http island pipeline
(SSRs + hydrates islands), not the http-listen host, and needs browser/Playwright
iteration which this worktree lacks. plans/blog-editor-island.md is the handoff:
goal, architecture (docs-side island -> host /new), the live host contract
(form-urlencoded title/sx_content/status -> 303), the sx_content markup to emit
(standard tags, NOT legacy ~kg-* cards), island authoring gotchas, and pointers.
Host side is ready (ingest proven; CORS on request). Phase 5.5 marked handed off.

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

plans/blog-editor-island.md

@@ -0,0 +1,75 @@
+# Handoff: native SX-island blog editor
+
+> Handed off from the **host-on-sx** loop (2026-06-19). Build this in a
+> **browser-capable session** (Playwright installed) — a reactive island only
+> proves out when it hydrates in a browser; this worktree has no Playwright.
+
+## Goal
+
+A native **SX reactive island** WYSIWYG block editor for blog posts — replacing
+the legacy `shared/static/scripts/sx-editor.js` (Koenig-era JS, ~2500 lines).
+It edits blocks reactively and, on publish, emits **`sx_content`** (SX element
+markup) + a title + status, and submits to the host's create endpoint.
+
+## Architecture (decided this session)
+
+- The editor is the **interactivity layer**, so it lives on the **`--http`
+  island pipeline** (`sx.rose-ash.com`, which already SSRs + hydrates islands),
+  **NOT** in the `http-listen` host (the host deliberately doesn't do island
+  hydration — see `plans/host-on-sx.md` Phase 5).
+- It **publishes to the host**: the host serves `blog.rose-ash.com` and owns the
+  durable store + create/render. The editor is a docs-side island that talks to
+  the host's API. Two cooperating SX servers: host = content/API/state, `--http`
+  = interactive UI.
+
+## The host contract (already live + proven)
+
+`POST /new` on the host (`blog.rose-ash.com`) — **works today**:
+- Body: **form-urlencoded** `title`, `sx_content`, `status` (`draft`/`published`).
+- Behaviour: slug derived from title, post stored in the durable KV, **303
+  redirect** to `/<slug>/`.
+- `host/blog-form-submit` in `lib/host/blog.sx`; route `host/blog-open-create-routes`
+  (currently UNGUARDED experimental — gate before real use).
+- A **form POST** (303 redirect) needs **no CORS**. If the editor uses `fetch`
+  instead, the host needs CORS on `/new` — the host loop can add `dream-cors-with`
+  (`lib/dream/cors.sx`) in minutes; just ask.
+
+## `sx_content` format — what to emit
+
+SX **element markup**, rendered host-side by `render-page` → `render-to-html`,
+**per block, guarded** (`host/blog-render` in `lib/host/blog.sx`). So:
+- Top level is a fragment: `(<> (h2 "Title") (p "para " (strong "bold")) (ul (li "a") (li "b")))`.
+- **Use standard tags `render-to-html` knows**: `p h1..h6 ul ol li blockquote
+  code pre strong em a img figure hr br span div`. These render cleanly + fast.
+- **AVOID the legacy `~kg-*` card components** — they show as `(unsupported
+  block)` placeholders (the legacy editor emits bare `~kg-md` but the components
+  are `~kg_cards/kg-md` — name drift we deliberately did NOT alias). If cards are
+  wanted, define **canonical** card components the host loads (no bare-name shim).
+- A bad/unknown block degrades to a placeholder, never crashes the page — but
+  aim to emit only renderable markup.
+
+## Build notes
+
+- It's a `defisland` served as a `defpage` on `--http`. Example island:
+  `sx/sx/home/stepper.sx`. Reactive primitives: `signal`/`deref`/`computed`/
+  `effect` (see the signals spec).
+- **SX island authoring gotchas** (CLAUDE.md "SX Island Authoring Rules"):
+  multi-expr bodies need `(do …)`; `let` is parallel (nest for sequencing);
+  reactive text needs `(deref (computed …))`; effects go in an inner `let`.
+- A reasonable MVP: title input (signal) + an ordered list of block signals
+  (type + text), add/remove/reorder, a few block types (paragraph, heading,
+  list, quote, code), a **live preview** (computed → rendered), and a Publish
+  that serialises blocks → `sx_content` and form-POSTs to the host's `/new`.
+- **Test with `sx_playwright`** (inspect / hydrate / interact / trace-boot) —
+  hydrate the island, simulate typing, assert the serialized `sx_content` and
+  the live preview. Don't ship an island you haven't hydrated in a browser.
+
+## Pointers
+
+- Host ingest + render + page shell: `lib/host/blog.sx` (the `/new` POST is the
+  target; `host/blog-render` shows exactly which markup renders).
+- `render-page` (host's component renderer) + the static-page pattern:
+  `lib/host/page.sx`, `plans/host-on-sx.md` Phase 5.
+- Island example: `sx/sx/home/stepper.sx`. HTML renderer (tags it knows):
+  `web/adapter-html.sx`. Legacy editor (reference only, being replaced):
+  `shared/static/scripts/sx-editor.js`.

plans/host-on-sx.md

@@ -251,10 +251,14 @@ Sub-steps (each independently gated/verified):
 - [ ] **5.4 Island hydration.** Confirm a trivial island page boots + hydrates
       client-side (sx-browser.js) when served by the host. Gate: a counter island
       increments in the browser.
-- [ ] **5.5 Editor POC.** Serve an editor page as the FIRST interactive consumer,
-      its form/island posting to the existing `POST /new` ingest (already proven).
-      Gate: visual edit → publish → renders, end-to-end in a browser. (The POC
-      validates the *capability*; the editor itself is replaced next — see below.)
+- [~] **5.5 Editor POC — HANDED OFF.** The native SX-island editor is the
+      interactivity layer; per the architecture it lives on the `--http` island
+      pipeline (not the host) and needs browser/Playwright iteration (absent in
+      this worktree). Handoff brief: `plans/blog-editor-island.md`. The host side
+      is READY: `POST /new` ingest is live + proven (form-urlencoded
+      title/sx_content/status → 303); CORS can be added on request if the editor
+      uses fetch. Decision: don't port island hydration into the host; the editor
+      is a docs-side island that publishes to the host.
 
 **Note:** component SSR is interpreted → slow until the `sx-vm-extensions` JIT
 loop lands; correctness first, speed follows. Scope spans `hosts/` (page-render