js-on-sx: raise JS TypeError for non-callable callee, undefined()

Calling a non-callable raised an OCaml-level Eval_error "Not callable"
that JS try/catch couldn't intercept. Added a (js-function? callable)
precheck in js-apply-fn that raises a TypeError instance via
(js-new-call TypeError (list msg)) so e instanceof TypeError is
true. Same swap for the undefined() branch in js-call-plain (was
raising a bare string). built-ins/String: 71/99 → 73/99 (canonical),
74/99 → 75/99 (isolated). conformance.sh: 148/148.

lib/js/test262-scoreboard.md

@@ -1,16 +1,15 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 375.3s
+Wall time: 280.3s
 
-**Total:** 71/99 runnable passed (71.7%). Raw: pass=71 fail=22 skip=1 timeout=6 total=100.
+**Total:** 73/99 runnable passed (73.7%). Raw: pass=73 fail=20 skip=1 timeout=6 total=100.
 
 ## Top failure modes
 
 - **10x** Test262Error (assertion failed)
 - **8x** TypeError: not a function
 - **6x** Timeout
-- **2x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)
 - **1x** SyntaxError (parse/unsupported syntax)
 
@@ -18,14 +17,14 @@ Wall time: 375.3s
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 71 | 22 | 1 | 6 | 100 | 71.7% |
+| built-ins/String | 73 | 20 | 1 | 6 | 100 | 73.7% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (71/99 — 71.7%)
+### built-ins/String  (73/99 — 73.7%)
 
 - **10x** Test262Error (assertion failed)
 - **8x** TypeError: not a function
 - **6x** Timeout
-- **2x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)
+- **1x** SyntaxError (parse/unsupported syntax)