cl: block + return-from — 13 new tests (140 eval, 312 total green)

Sentinel propagation in cl-eval-body; cl-eval-block catches matching
sentinels; BLOCK/RETURN-FROM/RETURN dispatch added to cl-eval-list.
Parser: CL strings now {:cl-type "string"} dicts for proper CL semantics.

lib/common-lisp/parser.sx

@@ -4,7 +4,7 @@
 ;;
 ;; AST representation:
 ;;   integer/float         → SX number (or {:cl-type "float"/:ratio ...})
-;;   string                → SX string
+;;   string "hello"        → {:cl-type "string" :value "hello"}
 ;;   symbol FOO            → SX string "FOO"  (upcase)
 ;;   symbol NIL            → nil
 ;;   symbol T              → true
@@ -96,7 +96,7 @@
             ((= type "integer") {:form (cl-convert-integer val) :rest nxt})
             ((= type "float") {:form {:cl-type "float" :value val} :rest nxt})
             ((= type "ratio") {:form {:cl-type "ratio" :value val} :rest nxt})
-            ((= type "string") {:form val :rest nxt})
+            ((= type "string") {:form {:cl-type "string" :value val} :rest nxt})
             ((= type "char") {:form {:cl-type "char" :value val} :rest nxt})
             ((= type "keyword") {:form {:cl-type "keyword" :name val} :rest nxt})
             ((= type "uninterned") {:form {:cl-type "uninterned" :name val} :rest nxt})