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/tests/control.sx

@@ -69,4 +69,47 @@
   (scm-ctl "(procedure? (call/cc (lambda (k) k)))")
   true)
 
+;; ── Exceptions: raise / guard / with-exception-handler / error ──
+(scm-ctl-test "raise + guard caught"
+  (scm-ctl "(guard (e (else 'caught)) (raise 'boom))") "caught")
+(scm-ctl-test "guard: number? matches"
+  (scm-ctl "(guard (e ((number? e) e) (else 'other)) (raise 42))") 42)
+(scm-ctl-test "guard: number? mismatches → else"
+  (scm-ctl "(guard (e ((number? e) e) (else 'other)) (raise 'sym))")
+  "other")
+(scm-ctl-test "guard: no error → body value"
+  (scm-ctl "(guard (e (else 'never)) 42)") 42)
+(scm-ctl-test "guard: first matching clause wins"
+  (scm-ctl
+    "(guard (e ((number? e) 'num) ((symbol? e) 'sym) (else 'other)) (raise 'foo))")
+  "sym")
+(scm-ctl-test "guard: re-raises when no clause matches"
+  (scm-ctl
+    "(guard (e (else 'outer)) (guard (e ((number? e) 'inner)) (raise 'not-a-number)))")
+  "outer")
+(scm-ctl-test "guard: var bound in clause body"
+  (scm-ctl "(guard (e ((symbol? e) e)) (raise 'the-symbol))")
+  "the-symbol")
+(scm-ctl-test "with-exception-handler: caught"
+  (scm-ctl
+    "(with-exception-handler (lambda (e) 'caught) (lambda () (raise 'oops)))")
+  "caught")
+(scm-ctl-test "with-exception-handler: no raise"
+  (scm-ctl
+    "(with-exception-handler (lambda (e) 99) (lambda () 42))")
+  42)
+(scm-ctl-test "with-exception-handler: handler sees the value"
+  (scm-ctl
+    "(with-exception-handler (lambda (e) (+ e 1)) (lambda () (raise 41)))")
+  42)
+(scm-ctl-test "error: irritants accessible"
+  (scm-ctl
+    "(guard (e ((error-object? e) (error-object-irritants e))) (error \"msg\" 1 2 3))")
+  (list 1 2 3))
+(scm-ctl-test "error: message accessible"
+  (scheme-string-value
+    (scm-ctl
+      "(guard (e ((error-object? e) (error-object-message e))) (error \"the-msg\"))"))
+  "the-msg")
+
 (define scm-ctl-tests-run! (fn () {:total (+ scm-ctl-pass scm-ctl-fail) :passed scm-ctl-pass :failed scm-ctl-fail :fails scm-ctl-fails}))