scheme: Phase 5b — R7RS exceptions (raise/guard/with-exception-handler) + 12 tests

eval.sx adds the `guard` syntactic operator with R7RS-compliant
clause dispatch: var binds to raised value in a fresh child env;
clauses tried in order; `else` is catch-all; no match re-raises.

Implementation uses a "catch-once-then-handle-outside" pattern to
avoid the handler self-raise loop:
  outcome = host-guard {body}            ;; tag raise vs success
  if outcome was raise:
    try clauses → either result or sentinel
    if sentinel: re-raise OUTSIDE the host-guard scope

runtime.sx binds R7RS exception primitives:
- raise V
- error MSG IRRITANT...  → {:scm-error MSG :irritants LIST}
- error-object?, error-object-message, error-object-irritants
- with-exception-handler HANDLER THUNK
  (same outcome-sentinel pattern — handler's own raises propagate
  outward instead of re-entering)

12 tests cover: catch on raise, predicate dispatch, else catch-all,
no-error pass-through, first-clause-wins, re-raise-on-no-match,
error-object construction and accessors.

232 total Scheme tests now (62 + 23 + 49 + 78 + 20).

lib/scheme/eval.sx

@@ -334,6 +334,77 @@
 (scheme-define-op! "or"
   (fn (args env) (scm-or-step args env)))
 
+;; ── guard (R7RS exception clause-dispatch syntactic form) ────────
+;; (guard (var (test1 body1) (test2 body2) ... [else body]) body...)
+;;
+;; Evaluates body in an exception-protected scope. If an exception is
+;; raised, var is bound to the raised value in a fresh child env, the
+;; cond-like clauses are tried in order, and the first matching clause's
+;; body is returned. If no clause matches (and no else), the exception
+;; re-raises. The bare `else` symbol is the catch-all per R7RS.
+
+;; Sentinel that means "no clause matched; re-raise outside the guard".
+(define scm-guard-no-match-marker {:scm-guard-no-match true})
+
+(define scm-guard-try-clauses
+  (fn (clauses env raised)
+    (cond
+      ((or (nil? clauses) (= (length clauses) 0))
+        scm-guard-no-match-marker)
+      (:else
+        (let ((clause (first clauses)))
+          (cond
+            ((not (list? clause)) scm-guard-no-match-marker)
+            ((and (string? (first clause)) (= (first clause) "else"))
+              (scheme-eval-body (rest clause) env))
+            (:else
+              (let ((test-val (scheme-eval (first clause) env)))
+                (cond
+                  ((not (= test-val false))
+                    (cond
+                      ((= (length clause) 1) test-val)
+                      (:else (scheme-eval-body (rest clause) env))))
+                  (:else
+                    (scm-guard-try-clauses (rest clauses) env raised)))))))))))
+
+(define scm-guard-handle
+  (fn (raised-val var-name clauses env)
+    (let ((local (scheme-extend-env env)))
+      (begin
+        (scheme-env-bind! local var-name raised-val)
+        (scm-guard-try-clauses clauses local raised-val)))))
+
+(scheme-define-op! "guard"
+  (fn (args env)
+    (cond
+      ((< (length args) 1)
+        (error "guard: expects ((var clauses...) body...)"))
+      ((not (list? (first args)))
+        (error "guard: first form must be (var clauses...)"))
+      ((= (length (first args)) 0)
+        (error "guard: clause list needs a var name"))
+      (:else
+        (let ((var-name (first (first args)))
+              (clauses (rest (first args)))
+              (body (rest args)))
+          ;; Catch once; if no clause matches, the sentinel is returned
+          ;; and we re-raise OUTSIDE the guard scope (so the re-raise
+          ;; doesn't itself get caught).
+          (let ((outcome
+                  (guard
+                    (e (true {:scm-guard-raised true :value e}))
+                    (scheme-eval-body body env))))
+            (cond
+              ((and (dict? outcome) (get outcome :scm-guard-raised))
+                (let ((result (scm-guard-handle (get outcome :value)
+                                                var-name clauses env)))
+                  (cond
+                    ((and (dict? result)
+                          (get result :scm-guard-no-match))
+                      (raise (get outcome :value)))
+                    (:else result))))
+              (:else outcome))))))))
+
 ;; ── eval-args helper ─────────────────────────────────────────────
 
 (define