Infer return types for block calls in returns and branches

[apiology]

Claude:

Problem: A method whose return value is a call with a block gets no inferred type when that call is the value of a return or of a conditional branch, so typecheck --level strong reports "return type could not be inferred" despite accurate @return tags upstream.

# @return [Array<Integer>]
def list; [1]; end

list.map { |x| x.to_s }          # => Array<String>
return list.map { |x| x.to_s }   # => undefined
return list.map(&:to_s)          # => Array<String>

if/else, unless/else, case/when, a ternary, and an if assigned to a local behave the same way; a block-pass or a call with no block does not.

Solution: reduce_to_value_nodes, which collects values from branches and from return, kept only the explicit returns inside a block body and discarded the block itself, where from_value_position_statement already treats a block as a value. It now pushes the block node too, additively: a return inside a block returns from the enclosing method, so both count.

Two @sg-ignore markers this obsoletes are removed, one recording that FlowSensitiveTyping#find_var had no inferrable return type. One is added where the new types reach a masgn call and expose a real narrowing gap, retiring the @todo there that asked for that alert. :numblock is untouched: NodeChainer recognises only :block.