Fix cross-file C namespace references

[sb123sb123]

Summary

• Resolve C namespace variables through the shared store when parsing later C files.
• Reuse the stored class or module object so constants and methods attach to the existing namespace.
• Add a two-file regression test covering both a constant and a method.

Fixes #1744.

Validation

• RUBYOPT=--enable-frozen_string_literal ruby -Ilib test/rdoc/parser/c_test.rb with Ruby 3.2.11 and Prism 1.7.0: 108 tests, 534 assertions, 0 failures or errors.
• ruby -c lib/rdoc/parser/c.rb
• ruby -c test/rdoc/parser/c_test.rb
• git diff --check
• The full test suite was not run locally.

AI assistance: Codex assisted with implementation and review.