host: Part A — type-block GRAMMAR (a Composition field declares which blocks it permits)

The type now GOVERNS the composition, not just declares the slot. A Composition field carries
its grammar: {:name "body" :type "Composition" :blocks (…card types…) :allow ("cond" "each")}.
:blocks absent -> any card subtype (back-compat); :allow absent -> both control blocks.

- host/blog--{field-decl, allowed-blocks, allows-control?, block-allowed?, comp-violations}.
- The editor PALETTE is the grammar: one <option> per allowed card type (spliced as direct
  <select> children), and the conditional/repeater add-forms appear only if :allow permits.
- block-add-submit ENFORCES it (was a coarse "any card subtype" check) — the type governs writes.
- comp-violations flags a composition holding a forbidden block (the save/import gate).
- article declares its :body grammar (all 7 card kinds + cond/each).

blog 179/179, full conformance 408/408 (+ grammar tests: allowed-blocks/allows-control?,
palette shows only permitted kinds, add rejects a forbidden card, violations flags one).
Part B (relations as type-governed composition) + Part C (edit the type definition) next.

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

lib/host/tests/blog.sx

@@ -934,6 +934,43 @@
 (host-bl-test "composition-fields defaults to [body] when the type declares none"
   (begin (host/blog-put! "plain1" "P" "(p)" "published") (host/blog--composition-fields "plain1"))
   (list "body"))
+;; -- LAYER 2b: type-block GRAMMAR — a Composition field declares which block kinds it PERMITS.
+;; The type governs the composition: editor palette + add + validation all read the grammar. --
+(host-bl-test "allowed-blocks + allows-control? read the field's grammar (:blocks / :allow)"
+  (begin
+    (host/blog-seed! "grmtype" "G" "(p)" "published")
+    (host/blog-relate! "grmtype" "type" "subtype-of")
+    (host/blog--set-fields! "grmtype"
+      (list {:name "body" :type "Composition" :blocks (list "card-text" "card-heading") :allow (list "cond")}))
+    (host/blog-put! "grm1" "G1" "(article)" "published")
+    (host/blog-relate! "grm1" "grmtype" "is-a")
+    (list (host/blog--allowed-blocks "grm1" "body")
+          (host/blog--block-allowed? "grm1" "body" "card-text") (host/blog--block-allowed? "grm1" "body" "card-image")
+          (host/blog--allows-control? "grm1" "body" "cond") (host/blog--allows-control? "grm1" "body" "each")))
+  (list (list "card-text" "card-heading") true false true false))
+;; the editor PALETTE is the grammar — only the allowed card types + permitted control blocks.
+(host-bl-test "the block editor palette shows only the grammar's card kinds + allowed controls"
+  (let ((html (render-page (host/blog--block-editor "grm1" "body"))))
+    (list (contains? html "value=\"card-text\"") (contains? html "value=\"card-heading\"")
+          (contains? html "value=\"card-image\"") (contains? html "+ conditional") (contains? html "+ repeater")))
+  (list true true false true false))
+;; the add handler REJECTS a card kind outside the grammar (the type governs writes).
+(host-bl-test "block-add-submit rejects a card kind the grammar forbids"
+  (begin
+    (host/blog--set-body! "grm1" (quote (seq)))
+    (host-bl-wapp (host-bl-send "POST" "/grm1/blocks/add" "Bearer good"
+      "application/x-www-form-urlencoded" "field=body&ctype=card-image&text=x"))
+    (list (len (host/blog--comp-nodes "grm1" "body"))                          ;; image rejected -> 0
+          (begin (host-bl-wapp (host-bl-send "POST" "/grm1/blocks/add" "Bearer good"
+                   "application/x-www-form-urlencoded" "field=body&ctype=card-text&text=ok"))
+                 (len (host/blog--comp-nodes "grm1" "body")))))                ;; text allowed -> 1
+  (list 0 1))
+;; validation flags a composition holding a block the grammar forbids (import/save gate).
+(host-bl-test "comp-violations flags a card kind outside the field's grammar"
+  (begin
+    (host/blog-block-add! "grm1" "body" "card-image" {"src" "x.jpg"})          ;; model add bypasses the handler guard
+    (> (len (host/blog--comp-violations "grm1" "body")) 0))
+  true)
 ;; the editor renders a HAND-AUTHORED composition (text/row/alt-with-text) WITHOUT falling
 ;; through to "(unknown block)" — every node kind gets a labelled row (the compose-demo case).
 (host-bl-test "the block editor renders text/layout/inline-alt nodes (no unknown block)"