Add one-line comments to all defines in 5 spec files

parser.sx (3), render.sx (15), harness.sx (21), signals.sx (23),
canonical.sx (12) — 74 comments total. Each define now has a ;;
comment explaining its purpose.

Combined with the evaluator.sx commit, all 215 defines across 6 spec
files are now documented. primitives.sx and special-forms.sx already
had :doc fields.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spec/parser.sx

@@ -50,6 +50,7 @@
 ;; --------------------------------------------------------------------------
 ;; Returns a list of top-level AST expressions.
 
+;; Parse SX source string into AST
 (define sx-parse :effects []
   (fn ((source :as string))
     (let ((pos 0)
@@ -360,6 +361,7 @@
 ;; Serializer — AST → SX source text
 ;; --------------------------------------------------------------------------
 
+;; Serialize AST value back to SX source
 (define sx-serialize :effects []
   (fn (val)
     (case (type-of val)
@@ -376,6 +378,7 @@
       :else       (str val))))
 
 
+;; Serialize a dict to SX {:key val} format
 (define sx-serialize-dict :effects []
   (fn ((d :as dict))
     (str "{"