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/runtime.sx

@@ -531,4 +531,59 @@
                     (scheme-apply (first args) (list scheme-k)))))))))
       (scheme-env-bind! env "call-with-current-continuation"
         (refl-env-lookup env "call/cc"))
+      ;; ── R7RS exception primitives ──────────────────────────
+      ;; raise V — raises V as exception (host SX raise).
+      (scheme-env-bind! env "raise"
+        (fn (args)
+          (cond
+            ((not (= (length args) 1))
+              (error "raise: expects 1 argument"))
+            (:else (raise (first args))))))
+      ;; error MSG IRRITANTS... — convention: raise an error-object
+      ;; that's a dict {:scm-error MSG :irritants LIST}. The print
+      ;; surface (error-object-message / error-object-irritants)
+      ;; can pull these apart.
+      (scheme-env-bind! env "error"
+        (fn (args)
+          (cond
+            ((= (length args) 0) (error "error: expects (message [irritant...])"))
+            (:else
+              (raise {:scm-error (cond
+                                   ((scheme-string? (first args))
+                                     (scheme-string-value (first args)))
+                                   (:else (first args)))
+                      :irritants (rest args)})))))
+      (scheme-env-bind! env "error-object?"
+        (scm-unary "error-object?"
+          (fn (v) (and (dict? v) (string? (get v :scm-error))))))
+      (scheme-env-bind! env "error-object-message"
+        (scm-unary "error-object-message"
+          (fn (v) (scheme-string-make (get v :scm-error)))))
+      (scheme-env-bind! env "error-object-irritants"
+        (scm-unary "error-object-irritants"
+          (fn (v) (get v :irritants))))
+      ;; with-exception-handler HANDLER THUNK — runs THUNK; if it
+      ;; raises, calls HANDLER with the raised value (handler can
+      ;; itself raise or return a value). Implemented via host guard.
+      ;; with-exception-handler — catch THUNK's raise; if caught,
+      ;; call HANDLER. If HANDLER itself raises, propagate that to
+      ;; the outer scope (don't re-catch in this same guard, which
+      ;; would loop). The two-step outcome-sentinel pattern mirrors
+      ;; the `guard` special form's escape.
+      (scheme-env-bind! env "with-exception-handler"
+        (fn (args)
+          (cond
+            ((not (= (length args) 2))
+              (error "with-exception-handler: expects 2 arguments"))
+            (:else
+              (let ((handler (first args))
+                    (thunk (nth args 1)))
+                (let ((outcome
+                        (guard
+                          (e (true {:scm-weh-raised true :value e}))
+                          (scheme-apply thunk (list)))))
+                  (cond
+                    ((and (dict? outcome) (get outcome :scm-weh-raised))
+                      (scheme-apply handler (list (get outcome :value))))
+                    (:else outcome))))))))
       env)))