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>

lib/host/sxtp.sx

@@ -4,10 +4,10 @@
 ;; at applications/sxtp/spec.sx.
 ;;
 ;; Representation: internally a message is a plain dict tagged by :msg ("request"
-;; /"response"/"condition"/"event"), with string keys so the keyword==string rule
-;; makes construction and access trivial. verb/status/type are stored as SYMBOLS
-;; (they ride the wire bare, not quoted). The wire LIST form is produced/consumed
-;; only at the serialise/parse boundary:
+;; /"response"/"condition"/"event"/"patch"/"signals"), with string keys so the
+;; keyword==string rule makes construction and access trivial. verb/status/type/
+;; mode are stored as SYMBOLS (they ride the wire bare, not quoted). The wire
+;; LIST form is produced/consumed only at the serialise/parse boundary:
 ;;   sxtp/serialize : msg-dict -> text/sx string
 ;;   sxtp/parse     : text/sx string -> msg-dict
 ;; A Dream HTTP request/response bridges to/from SXTP via sxtp/from-dream and
@@ -35,6 +35,21 @@
   (fn (etype opts)
     (merge {:msg "event" :type (sxtp/-sym etype)} opts)))
 
+;; Patch (Datastar-borrowed) — DOM fragment morph.
+;; target: CSS selector (required). mode in opts defaults to outer; accepts
+;; string OR symbol and is normalised. mode values: outer | inner | replace |
+;; prepend | append | before | after | remove. body: SX subtree (omit for remove).
+(define sxtp/patch
+  (fn (target opts)
+    (let ((mode (or (get opts :mode) "outer")))
+      (merge opts {:msg "patch" :target target :mode (sxtp/-sym mode)}))))
+
+;; Signals (Datastar-borrowed) — reactive state patch.
+;; values: dict of signal-name -> new-value (nil removes). only-if-missing: bool.
+(define sxtp/signals
+  (fn (values opts)
+    (merge {:msg "signals" :values values} opts)))
+
 ;; ── predicates ─────────────────────────────────────────────────────
 (define sxtp/-is?
   (fn (m tag) (and (= (type-of m) "dict") (= (get m :msg) tag))))
@@ -42,6 +57,8 @@
 (define sxtp/response? (fn (m) (sxtp/-is? m "response")))
 (define sxtp/condition? (fn (m) (sxtp/-is? m "condition")))
 (define sxtp/event? (fn (m) (sxtp/-is? m "event")))
+(define sxtp/patch? (fn (m) (sxtp/-is? m "patch")))
+(define sxtp/signals? (fn (m) (sxtp/-is? m "signals")))
 
 ;; ── accessors ──────────────────────────────────────────────────────
 (define sxtp/verb (fn (m) (get m :verb)))
@@ -56,6 +73,11 @@
 (define sxtp/stream? (fn (m) (= (get m :stream) true)))
 (define sxtp/cond-type (fn (m) (get m :type)))
 (define sxtp/cond-message (fn (m) (get m :message)))
+(define sxtp/target (fn (m) (get m :target)))
+(define sxtp/mode (fn (m) (get m :mode)))
+(define sxtp/values (fn (m) (get m :values)))
+(define sxtp/only-if-missing? (fn (m) (= (get m :only-if-missing) true)))
+(define sxtp/transition? (fn (m) (= (get m :transition) true)))
 
 ;; ── status helpers (build responses) ───────────────────────────────
 (define sxtp/ok (fn (body) (sxtp/response "ok" {:body body})))
@@ -121,7 +143,9 @@
   {:request (list :verb :path :headers :cookies :params :capabilities :body)
    :response (list :status :headers :set-cookie :body :stream)
    :condition (list :type :message :path :retry :detail)
-   :event (list :type :id :body :time)})
+   :event (list :type :id :body :time)
+   :patch (list :target :mode :body :transition)
+   :signals (list :values :only-if-missing)})
 ;; A nested SXTP message (a condition/event in a :body) serialises in its own
 ;; list form; plain data values go through the serialize primitive.
 (define sxtp/-emit-value