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, 1 insertions, 1 deletions

diff --git a/tests/test-mail-config-transport.el b/tests/test-mail-config-transport.el
index 2244b6dd..0240102a 100644
--- a/tests/test-mail-config-transport.el
+++ b/tests/test-mail-config-transport.el
@@ -18,7 +18,7 @@ EXECUTABLES is an alist of program name strings to executable paths."
   (declare (indent 1))
   `(let (test-mail-config--warnings)
      (cl-letf (((symbol-function 'executable-find)
-                (lambda (program)
+                (lambda (program &rest _)
                   (cdr (assoc program ,executables))))
                ((symbol-function 'display-warning)
                 (lambda (type message &rest _args)