Add pattern matching type inference

[dak2]

Motivation

Pattern matching was not supported by MethodRay, causing it to silently skip type inference for matched variables.

Changes

• Add CaseMatchNode processing with support for all pattern types: capture, array, hash, find, alternation, pinned variable/expression, guard (if), and literal
• Fix hash shorthand { name: } binding via ImplicitNode unwrapping
• Fix capture pattern to handle namespaced constants (Api::User => u) via extract_constant_path
• Fix hash splat **rest to read from HashPatternNode.rest() instead of elements()
• Unify all variable bindings through install_local_var_write for consistent ChangeSet edge propagation
• Remove unreachable UnlessNode guard branch (Ruby does not support unless guards in case/in)

Checked

• cargo test --lib (44 passed)
• bundle exec ruby -Ilib -Itest test/pattern_matching_test.rb (18 passed)