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-music-config-playlist-commands.el b/tests/test-music-config-playlist-commands.el
index 3d6dfd8b..891bc700 100644
--- a/tests/test-music-config-playlist-commands.el
+++ b/tests/test-music-config-playlist-commands.el
@@ -132,7 +132,7 @@
         (cl-letf (((symbol-function 'cj/music--playlist-modified-p)
                    (lambda () nil))
                   ((symbol-function 'find-file-other-window)
-                   (lambda (p) (setq opened p))))
+                   (lambda (p &rest _) (setq opened p))))
           (cj/music-playlist-edit))
         (should (equal opened tmp))
         (delete-file tmp))