fix: add Mutex_m RBS type stubs for validation

The rbs-inline tool generates 'include Mutex_m' in the type
signatures for Bayes and LSI classes, but RBS validation fails
because it cannot find the Mutex_m type definition.

Add vendor type stubs with the core mutex methods and their
standard aliases (synchronize, lock, unlock, etc.) to satisfy
both RBS validation and Steep type checking.

Author: cardmagic

sig/vendor/mutex_m.rbs

@@ -0,0 +1,16 @@
+# Type stubs for mutex_m gem
+module Mutex_m
+  def mu_initialize: () -> void
+  def mu_lock: () -> void
+  def mu_unlock: () -> void
+  def mu_synchronize: [T] () { () -> T } -> T
+  def mu_try_lock: () -> bool
+  def mu_locked?: () -> bool
+
+  # Aliases
+  alias lock mu_lock
+  alias unlock mu_unlock
+  alias synchronize mu_synchronize
+  alias try_lock mu_try_lock
+  alias locked? mu_locked?
+end