core: syntax tier — move when to the overlay ahead of the kernel (jolt-1j0 phase 3)

New 00-syntax tier loaded FIRST (before 00-kernel), interpreted defmacros, so the
control macros the compiler and every later tier depend on can live in Clojure.
Validated by moving when: the kernel tier, self-hosted analyzer and seq/coll
tiers all compile against the overlay when. Constraint: syntax-tier macros may use
only special forms + core-renames seed primitives (not second/peek/kernel fns).

conformance 228/228 x3, full suite green.

jolt-core/clojure/core/00-syntax.clj

@@ -0,0 +1,13 @@
+;; clojure.core — syntax tier. The control macros the compiler and every later
+;; tier depend on (when/cond/and/or/...), expressed as defmacro. Loaded FIRST
+;; (before 00-kernel), interpreted, so the macros exist before any code that uses
+;; them is compiled — including the kernel tier, the self-hosted analyzer, and the
+;; seq/coll tiers.
+;;
+;; CONSTRAINT: a macro here may use ONLY special forms (if/do/let*/fn*/not) and
+;; core-renames SEED primitives (first/next/rest/nth/count/empty?/...). It must
+;; NOT use kernel-tier fns (second/peek/subvec/...) or anything defined later —
+;; those don't exist yet when this tier loads.
+
+(defmacro when [test & body]
+  `(if ~test (do ~@body)))

src/jolt/api.janet

@@ -37,7 +37,8 @@
 # source (compiled when :compile?, interpreted otherwise — the analyzer, built
 # lazily on the first such form, sees the kernel tier already in place).
 (def- core-tiers
-  [{:ns "clojure.core.00-kernel" :kernel true}
+  [{:ns "clojure.core.00-syntax" :kernel false}
+   {:ns "clojure.core.00-kernel" :kernel true}
    {:ns "clojure.core.10-seq"    :kernel false}
    {:ns "clojure.core.20-coll"   :kernel false}
    {:ns "clojure.core.30-macros" :kernel false}])

src/jolt/core.janet

@@ -2092,14 +2092,6 @@
             (build (tuple/slice cls 2))]))))
   @[{:jolt/type :symbol :ns nil :name "let*"} @[g expr] (build clauses)])
 
-(defn core-when
-  "Macro: (when test & body) -> (if test (do body...))"
-  [test & body]
-  (def arr (array ;body))
-  (array/insert arr 0 {:jolt/type :symbol :ns nil :name "do"})
-  @[{:jolt/type :symbol :ns nil :name "if"}
-    test
-    arr])
 
 (defn core-when-not
   "Macro: (when-not test & body) -> (when (not test) & body)"
@@ -3598,7 +3590,6 @@
     "cond" core-cond
     "case" core-case
     "for" core-for
-    "when" core-when
     "when-not" core-when-not
     "when-let" core-when-let
     "->" core-thread-first
@@ -3684,7 +3675,7 @@
 (defn core-macro-names
   "Set of core binding names that are macros."
   []
-  @{"and" true "or" true "cond" true "case" true "for" true "when" true "when-not" true "when-let" true "defn" true "defn-" true "declare" true "fn" true "let" true "loop" true "defrecord" true "defprotocol" true "extend-type" true "extend-protocol" true "extend" true "reify" true "proxy" true "definterface" true "lazy-seq" true "lazy-cat" true "cond->" true "->" true "->>" true "doseq" true})
+  @{"and" true "or" true "cond" true "case" true "for" true "when-not" true "when-let" true "defn" true "defn-" true "declare" true "fn" true "let" true "loop" true "defrecord" true "defprotocol" true "extend-type" true "extend-protocol" true "extend" true "reify" true "proxy" true "definterface" true "lazy-seq" true "lazy-cat" true "cond->" true "->" true "->>" true "doseq" true})
 
 (def init-core!
   (fn [& args]