Don't crash on Ruby 4.1, which removed ObjectSpace._id2ref

objectspace/op_fetch.rb aliases ObjectSpace[] to _id2ref, which Ruby head
(4.1) no longer has. The alias raised NameError at load time, and since
facets/objectspace loads it, so did `require 'facets'`. The unit tests
never load that file, so only the new qed step caught it.

ObjectSpace[] is now only defined where _id2ref exists.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2Njj4j1r9twkVcXsMySLW

Author: claude

HISTORY.md

@@ -38,9 +38,13 @@ Changes:
     quietly changed both.
   * `Enumerable::Argumentable#min` and `#max` now pass their arguments to
     `#each`, as documented, rather than to Ruby's own `min(n)`/`max(n)`.
+  * `require 'facets'` no longer crashes on Ruby 4.1, which removed
+    `ObjectSpace._id2ref`. `ObjectSpace[]` is only defined where `_id2ref`
+    exists.
 
 * Internal
 
+  * Run the qed demos in CI.
   * Fix the qed demos, which had 8 failures and 2 errors. Most were stale
     expectations (`Array#from`, `#step`, `#arrange`, `String#similarity`)
     and the date demo held pasted irb output.

lib/core/facets/objectspace/op_fetch.rb

@@ -1,3 +1,4 @@
 class << ObjectSpace
-  alias_method :[], :_id2ref
+  # Ruby 4.1 removed ObjectSpace._id2ref, so there is nothing to alias there.
+  alias_method :[], :_id2ref if method_defined?(:_id2ref)
 end