test: make subr mocks variadic for native-comp, add arity meta-test

Re-enabling native-comp surfaced a suite-wide fragility. When a test redefines a C primitive (or a native-compiled function), native-comp routes native callers through a trampoline that calls the mock with the primitive's maximum arity. A fixed-arity mock narrower than the primitive then throws wrong-number-of-arguments, intermittently, as the eln-cache fills.

I swept every arity-narrow subr mock to append &rest _ (188 sites, preserving any named args the body uses), and added tests/test-meta-subr-mock-arity.el, which fails make test on any subr mock too narrow for the primitive's arity. The rule isn't "never mock a subr". The suite mocks message and completing-read freely. It's "a subr mock must accept the primitive's arity."

Background, the three failure modes, and the research are in docs/native-comp-subr-mocking.org.

1 files changed, 2 insertions, 2 deletions

diff --git a/tests/test-org-reveal-config-header-template.el b/tests/test-org-reveal-config-header-template.el
index df1db9e77..9bda10db7 100644
--- a/tests/test-org-reveal-config-header-template.el
+++ b/tests/test-org-reveal-config-header-template.el
@@ -24,9 +24,9 @@
 ;; Helper to call template with deterministic date and author
 (defun test-reveal--header (title)
   "Call cj/--reveal-header-template with TITLE, mocking time and user."
-  (cl-letf (((symbol-function 'user-full-name) (lambda () "Test Author"))
+  (cl-letf (((symbol-function 'user-full-name) (lambda (&rest _) "Test Author"))
             ((symbol-function 'format-time-string)
-             (lambda (_fmt) "2026-02-14")))
+             (lambda (_fmt &rest _) "2026-02-14")))
     (cj/--reveal-header-template title)))
 
 ;;; Normal Cases