Preserve runtime errors over the statement-terminator check

[Bascy]

Summary

When a native (C) function returns an error mid-expression (e.g. myCFunc(badArg) * 60;), the engine currently reports "; expected" instead of the original error message.

Reproducing

static jsval_t bad(struct js *js, jsval_t *args, int nargs) {
  (void) args; (void) nargs;
  return js_mkerr(js, "boom");
}
...
js_set(js, js_glob(js), "bad", js_mkfun(bad));
js_eval(js, "bad() * 2;", ~0UL);
// Expected: "ERROR: boom"
// Actual:   "ERROR: ; expected"

Root cause

1. The native function calls js_mkerr(...) which sets js->tok = TOK_EOF and js->pos = js->clen to short-circuit further parsing.
2. do_call_op restores the parser state after the call returns (so the parser is sitting on the * token after )).
3. RTL_BINOP / LTR_BINOP short-circuit on is_err(res) and do not consume the trailing operator.
4. js_stmt's terminator check

   if (next(js) != TOK_SEMICOLON && next(js) != TOK_EOF && next(js) != TOK_RBRACE)
     return js_mkerr(js, "; expected");

   does not gate on is_err(res), sees the unconsumed *, and overwrites the real error (errmsg is a single shared buffer).

Fix

Short-circuit in js_stmt when res is already an error. One-line change in elk.c; existing terminator behavior for non-error statements is unchanged.

Test plan

• Added two unit tests in test_c_funcs covering C-function errors embedded in larger expressions (gt() * 2, 1 + gt(null, 1)).
• Existing tests for "; expected" (e.g. "1 2", "1 + 2 3", "{1}", function(){1}) still pass — they exercise expression-level non-error results followed by stray tokens, which is unaffected by this patch.
• CI run against full test matrix (gcc, g++, MSVC, MinGW).