sxtp: patch + signals primitives (Datastar-borrowed)

Adds two new top-level SXTP message types alongside
request/response/condition/event, modelled on Datastar's
datastar-patch-elements and datastar-patch-signals SSE events:

  (patch :target "#x" :mode outer :body (~card)) - DOM fragment
    morph. Subsumes HTMX swap modes. Mode is outer (default) |
    inner | replace | prepend | append | before | after | remove.

  (signals :values {:n 3} :only-if-missing false) - reactive
    state patch. nil value removes the signal. only-if-missing
    skips existing signals (lazy init).

A server response stream can mix both freely; clients dispatch
by head symbol, ordering preserved. Cleaner than HTMX's
swap-mode-per-trigger because the patch shape is decoupled from
the triggering element/attribute.

Spec at applications/sxtp/spec.sx (patch-fields, signals-fields,
patch-modes, example-patch-stream). Constructors / predicates /
accessors / serialise / parse in lib/host/sxtp.sx. 25 new tests
in lib/host/tests/sxtp.sx (predicates, mode normalisation, fixed
field order, remove-without-body, signals round-trip). Host
conformance 129/129 (was 104/104).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

applications/sxtp/spec.sx

@@ -97,6 +97,42 @@
       (:body "Any SX value — event payload (optional)")
       (:time "Number — unix timestamp (optional)"))))
 
+;; ── patch (DOM fragment patch — borrowed from Datastar) ───────────
+;; A server-driven instruction to morph a region of the client DOM.
+;; Subsumes HTMX swap modes; the :body is an SX subtree that the client
+;; renders to DOM nodes before applying the mode at the target.
+(define
+  patch-fields
+  (quote
+    ((:target "String — CSS selector for the element to patch (required)")
+      (:mode "Symbol — patch mode (optional, default outer)")
+      (:body "SX tree — the new content (omitted for mode remove)")
+      (:transition "Boolean — use a view transition (optional, default false)"))))
+
+(define
+  patch-modes
+  (quote
+    ((outer "Replace the target's outerHTML (default; the morph target)")
+      (inner "Replace the target's innerHTML, preserving the wrapper")
+      (replace "Hard-replace without morphing (no diff, plain swap)")
+      (prepend "Insert the body as the target's first child")
+      (append "Insert the body as the target's last child")
+      (before "Insert the body before the target")
+      (after "Insert the body after the target")
+      (remove "Detach the target; :body MUST be absent"))))
+
+;; ── signals (reactive state patch — borrowed from Datastar) ──────
+;; A server-driven update to client-side reactive signals. :values is a
+;; dict of signal-name -> new-value; setting a value to nil REMOVES the
+;; signal. With :only-if-missing true, existing signals are not touched
+;; (use this to lazily initialise signal state without clobbering).
+(define
+  signals-fields
+  (quote
+    ((:values "Dict — signal-name -> new-value (required)")
+      (:only-if-missing
+        "Boolean — only set signals that don't yet exist (optional, default false)"))))
+
 (define
   example-navigate
   (quote
@@ -148,6 +184,23 @@
           :message "No such post"
           :retry false)))))
 
+;; A streaming response intermixing patch + signals: the server pushes
+;; DOM updates AND signal updates over the same channel. The client
+;; dispatches each message by its head symbol; ordering is preserved.
+(define
+  example-patch-stream
+  (quote
+    ((request :verb subscribe :path "/cart/live" :capabilities (fetch))
+      (response :status ok :stream true)
+      (signals :values {:cart/count 3 :cart/loading false})
+      (patch
+        :target "#cart-mini"
+        :mode outer
+        :body (~cart-mini :count 3 :total 47.50))
+      (patch :target "#flash" :mode inner :body (p "Item added."))
+      (signals :values {:cart/loading true})
+      (patch :target "#cart-loading-spinner" :mode remove))))
+
 (define
   example-inspect
   (quote