vm-ext: gate serving-JIT behind SX_SERVING_JIT + fix continuation-guest regressions

Enabling the epoch serving-mode JIT globally regressed continuation-based guest
interpreters (the epoch mode is the shared command channel every loop's
conformance runner uses). Two-part fix:

1. SAFE DEFAULT GATE. register_jit_hook in the persistent server branch is now
   opt-in via SX_SERVING_JIT=1 (default OFF). Default behaviour is unchanged
   (no JIT in epoch serving) → zero regression for sibling loops. The
   content/Smalltalk page server opts in.

2. GENERAL FIXES + per-guest interpret-only declarations:
   - callable? (sx_server/run_tests/integration_tests/mcp_tree) now accepts
     VmClosure. A JIT-compiled higher-order function returns its inner closure
     as a VmClosure; callable? previously rejected it, so scheme-apply's
     (callable? proc) guard failed with "not a procedure: <vm:anon>".
   - jit-exclude! gains a trailing-"*" namespace-prefix form
     (Sx_types.jit_excluded_prefixes), the robust way to mark a whole guest
     interpreter interpret-only (a name-list misses functions in extra files —
     it left erlang's vm/dispatcher JIT'd and 13 tests short).
   - Per-guest exclusions in each guest's runtime.sx:
       scheme  "scheme-*" "scm-*"   erlang "er-*" "erlang-*"
       prolog  "pl-*"               common-lisp "cl-*" "clos-*"
       js      "js-*"               haskell "hk-*"

Verified under opt-in JIT (== CEK, no hang): smalltalk 847/847, scheme/flow
166/166, erlang 530/530, prolog 590/590, apl 152/152, js 147/148. Residual
(documented, protected by the default gate): common-lisp 6 fails in advanced
suites (parser-recovery/debugger/CLOS/MOP). lua (0/16) and tcl (3/4) fail
identically on CEK — pre-existing, not JIT. run_tests --jit/no-jit unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/common-lisp/runtime.sx

@@ -757,4 +757,10 @@
                    "format-arguments" args))))
       (cl-restart-case
         (fn () (cl-signal-obj obj cl-handler-stack))
-        (list "continue" (list) (fn () nil))))))
+        (list "continue" (list) (fn () nil))))))
+;; ── JIT interpret-only boundary ───────────────────────────────────────────
+;; The Common-Lisp evaluator implements block/return-from, catch/throw, and
+;; the condition system via non-local control (host continuations); under JIT
+;; a compiled frame can't transfer control through a CEK continuation. Exclude
+;; the cl-/clos- namespaces from JIT. See Sx_types.jit_excluded_prefixes.
+(jit-exclude! "cl-*" "clos-*")