compiler: lower syntax-quote to construction code so backtick compiles

Following the canonical read->macroexpand->compile model (Clojure LispReader /
tools.reader): a syntax-quote is lowered to plain construction code instead of
being interpreted. Adds form builders (__sqcat -> array, __sqvec -> tuple,
__sqmap, __sq1) and syntax-quote-lower (mirrors the interpreter's syntax-quote*/
sq-symbol exactly: foo# auto-gensym, core/special unqualified, else qualify;
~ -> expr, ~@ -> splice). The analyzer now handles syntax-quote as a special:
lower then analyze the result. The interpreter keeps the self-contained
syntax-quote* (no core dependency, works in a bare ctx); the two are cross-checked
by conformance interpret-vs-compile.

Effect: a backtick body compiles (1.96s -> 1.19s/200k) instead of falling back to
the interpreter. Next: compile defmacro expanders for the full macro speedup.

conformance 228/228 x3, full suite green.

jolt-core/jolt/analyzer.clj

@@ -22,12 +22,13 @@
                                form-literal? form-elements form-vec-items
                                form-map-pairs form-special? compile-ns
                                form-macro? form-expand-1 resolve-global
-                               form-sym-meta host-intern!]]))
+                               form-sym-meta host-intern! form-syntax-quote-lower]]))
 
 (declare analyze)
 
 (def ^:private handled
-  #{"quote" "if" "do" "def" "fn*" "let*" "loop*" "recur" "throw" "try"})
+  #{"quote" "if" "do" "def" "fn*" "let*" "loop*" "recur" "throw" "try"
+    "syntax-quote"})
 
 (defn- uncompilable [why]
   (throw (str "jolt/uncompilable: " why)))
@@ -159,6 +160,9 @@
                :args (mapv #(analyze ctx % env) (rest items))})
     "try" (analyze-try ctx items env)
     "fn*" (analyze-fn ctx items env)
+    ;; Lower the backtick to construction code (zero runtime cost), then analyze
+    ;; it — the macroexpand/compile-time step, per read -> macroexpand -> compile.
+    "syntax-quote" (analyze ctx (form-syntax-quote-lower ctx (second items)) env)
     (uncompilable (str "special form " op))))
 
 (defn- analyze-symbol [ctx form env]

src/jolt/core.janet

@@ -124,6 +124,26 @@
       items)
     c))
 
+# Syntax-quote form builders. The syntax-quote lowering (evaluator) emits calls to
+# these so a `(...)/`[...] body is plain compilable code instead of an interpreted
+# special form. A list FORM is a Janet array, a vector FORM a tuple (the reader's
+# representation), so these build those types. Each concat part is either a 1-elem
+# wrap (__sq1, a non-spliced item) or a spliced seq (~@), flattened in order.
+(defn core-sq1 [x] @[x])
+
+(defn core-sqcat [& parts]
+  (def r @[])
+  (each p parts (each x (realize-for-iteration p) (array/push r x)))
+  r)
+
+(defn core-sqvec [& parts]
+  (def r @[])
+  (each p parts (each x (realize-for-iteration p) (array/push r x)))
+  (tuple/slice r))
+
+# Map builder: parts are alternating k v (no splicing in map syntax-quote).
+(defn core-sqmap [& parts] (kvs->map (array ;parts)))
+
 # ============================================================
 # Predicates
 # ============================================================
@@ -3289,6 +3309,10 @@
     "cons" core-cons
     "seq" core-seq
     "vec" core-vec
+    "__sq1" core-sq1
+    "__sqcat" core-sqcat
+    "__sqvec" core-sqvec
+    "__sqmap" core-sqmap
     "into" core-into
     "merge" core-merge
     "merge-with" core-merge-with

src/jolt/evaluator.janet

@@ -163,6 +163,49 @@
       (array/push kvs (syntax-quote* ctx bindings (get form k) gsmap))) (struct ;kvs))
     form))
 
+# Syntax-quote LOWERING: instead of evaluating a `(...) form to a value (what
+# syntax-quote* does), produce equivalent CONSTRUCTION CODE so a backtick body is
+# plain compilable code (read -> macroexpand -> compile, zero runtime cost).
+# Mirrors syntax-quote*/sq-symbol exactly; the canonical algorithm is
+# tools.reader's syntax-quote*/expand-list. List forms build via __sqcat (-> array),
+# vectors via __sqvec (-> tuple), maps via __sqmap; symbols become (quote resolved);
+# ~ leaves the expr in place, ~@ passes the seq straight to __sqcat for splicing.
+(defn- sqsym* [nm] {:jolt/type :symbol :ns nil :name nm})
+
+(var syntax-quote-lower nil)
+
+(defn- sq-lower-part [ctx item gsmap]
+  (if (and (array? item) (> (length item) 0) (sym-name? (first item) "unquote-splicing"))
+    (in item 1)
+    @[(sqsym* "__sq1") (syntax-quote-lower ctx item gsmap)]))
+
+(set syntax-quote-lower
+  (fn syntax-quote-lower [ctx form &opt gsmap]
+    (default gsmap @{})
+    (cond
+      (and (array? form) (> (length form) 0) (sym-name? (first form) "unquote"))
+      (in form 1)
+      (and (array? form) (> (length form) 0) (sym-name? (first form) "unquote-splicing"))
+      (error "~@ used outside of a list or vector in syntax-quote")
+      (or (number? form) (string? form) (keyword? form) (nil? form) (= true form) (= false form))
+      form
+      (and (struct? form) (= :symbol (form :jolt/type)))
+      @[(sqsym* "quote") (sq-symbol ctx form gsmap)]
+      (array? form)
+      (array/concat @[(sqsym* "__sqcat")] (map (fn [it] (sq-lower-part ctx it gsmap)) form))
+      (tuple? form)
+      (array/concat @[(sqsym* "__sqvec")] (map (fn [it] (sq-lower-part ctx it gsmap)) form))
+      # tagged structs (sets/chars): syntax-quote* returns them as-is (no recursion)
+      (and (struct? form) (get form :jolt/type))
+      @[(sqsym* "quote") form]
+      (struct? form)
+      (do (var parts @[(sqsym* "__sqmap")])
+          (each k (keys form)
+            (array/push parts (syntax-quote-lower ctx k gsmap))
+            (array/push parts (syntax-quote-lower ctx (get form k) gsmap)))
+          parts)
+      @[(sqsym* "quote") form])))
+
 (defn resolve-var
   [ctx bindings sym-s]
   (let [name (sym-s :name) ns (sym-s :ns)]
@@ -621,6 +664,10 @@
               nil))
   (match name
     "quote" (in form 1)
+    # Interpreter builds the form directly (self-contained, no core dependency).
+    # The COMPILE path instead lowers syntax-quote to construction code (via
+    # syntax-quote-lower) so a backtick body is compilable; the two are kept in
+    # sync and cross-checked by conformance (interpret vs compile modes).
     "syntax-quote" (syntax-quote* ctx bindings (in form 1))
     "unquote" (error "Unquote not valid outside of syntax-quote")
     "unquote-splicing" (error "Unquote-splicing not valid outside of syntax-quote")

src/jolt/host_iface.janet

@@ -143,6 +143,11 @@
 # Form predicates use `form-*` names (not list?/vector?/map?/set?/char?) so the
 # analyzer can refer them unqualified without the bootstrap's core-renames
 # intercepting them as the value-level predicates.
+# Lower a syntax-quote's inner form to construction code (so the analyzer can
+# compile it). The portable analyzer calls this and analyzes the result.
+(defn h-syntax-quote-lower [ctx inner]
+  (syntax-quote-lower ctx inner))
+
 (def- exports
   {"form-sym?" h-sym? "form-sym-name" h-sym-name "form-sym-ns" h-sym-ns
    "form-sym-meta" h-sym-meta
@@ -152,6 +157,7 @@
    "form-map-pairs" h-map-pairs "form-set-items" h-set-items
    "form-special?" h-special? "compile-ns" h-current-ns "form-macro?" h-macro?
    "form-expand-1" h-expand-1 "resolve-global" h-resolve-global
+   "form-syntax-quote-lower" h-syntax-quote-lower
    "host-intern!" h-intern!})
 
 (defn install! [ctx]