Support type inference for pattern-matching capture variables

[pvcresin]

Summary

Capture variables in case/in patterns were always inferred as untyped because the matched value was never threaded into the patterns:

case a
in Integer => n
  n + 1        # n was untyped
in [String => s, Symbol => t]
  [s, t]       # s, t were untyped
end

Changes

• Add an install_pattern / install_pattern0 pair (mirroring install / install0, so @changes is reconciled the same way during incremental analysis) that flows the matched value into each pattern node.
• Local targets bind to the matched value; Const => var narrows it by the class the same way when Const does (reusing IsAFilter); array / find patterns decompose it into element vertices via splat boxes. CaseMatchNode and the one-line => / in forms pass the subject in.
• Hash patterns still capture as untyped (extracting a value type per key needs a dedicated box); sub-patterns inside them are still matched.
• Promote scenario/known-issues/pattern-capture-narrowing.rb into scenario/patterns/capture.rb and update scenario/patterns/var_pat.rb.

All 433 tests pass (bundle exec rake test).