Move conditional from Region to the CompoundStatement pin itself

[apiology]

Follow-up to castwide#1282 (via #57), stacked on the same head branch.

Summary

Region#conditional was a separate boolean threaded alongside compound_statement, requiring every node processor to pass both in lockstep (e.g. block_node.rb: compound_statement: block_pin, conditional: true). Keeping two parallel values in sync at every call site is exactly the kind of duplication #57 set out to remove, and it's the shape of bug that broke Block handling mid-refactor there (definite was briefly, incorrectly, derived from compound_statement.is_a?(Closure), which is true for Block despite a block body running zero or many times).

conditional is now a constructor attribute on Pin::CompoundStatement itself, set once where each construct is built (Pin::Block.new(..., conditional: true), Pin::Method.new(...) defaulting false), so there's only one thing to get right per site instead of two.

It can't be a class-level constant: the bare Pin::CompoundStatement class is used both for an if's own condition (never conditional) and for then/else/rhs/rescue bodies (always conditional) - same class, different instances, different answers - so it stays an instance attribute, same as closure:/compound_statement: already are.

lvasgn_node.rb's definite computation becomes a single-hop read: !region.compound_statement.conditional, no separate Region field. Pin::CompoundStatement#combine_with merges the new attribute via choose, since two versions of the same construct should already agree on it.

Test plan

• bundle exec rspec spec/ - 1638 examples, 0 failures
• bundle exec solargraph typecheck --level strong - diffed clean against the prior baseline (587 problems, unchanged)
• bundle exec rubocop - clean on touched files

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com