Keep a block's reassignment of an outer local visible after the block

[apiology]

A local assigned inside a block is invisible once the block ends, so code that
is correct at runtime reports an unresolved call:

# @return [Array, nil]
def collect(lines)
  chomped = nil
  lines.each { |line| chomped = [line.chomp] }
  chomped   # inferred `nil`, so `chomped.reject` is unresolved
end

The same assignment inside an if already infers the union nil, ::Array.
Only the block form loses it.

Cause. LvasgnNode#process set the pin's presence to
region.closure.location.range.ending. Inside a block that closure is the
Pin::Block, so presence stops at the block's closing token:

chomped @3:4  closure=Pin::Method  presence=3,4..8,5
chomped @5:6  closure=Pin::Block   presence=5,6..6,7   <- invisible at line 7

Fix. LvasgnNode walks outward for an enclosing closure that already has
that name in scope at that position, and attaches the pin there. A name the
block introduces itself ��� block parameter, shadow arg, or first assignment in
the block ��� keeps the block closure and still shadows correctly.

This deliberately does not silence downstream nil errors. Post-block inference
becomes nil, ::Array, not ::Array, because the block may run zero times;
chomped.reject on that union remains correctly unresolved.

Adds 5 examples in spec/pin/local_variable_spec.rb.

Measured against 8fda63384: rubocop 70 offences before and after, output
byte-identical; self-hosted typecheck --level strong 525 problems in 89 of
250 files before and after; rspec unchanged at 1624 examples / 1 pre-existing
failure / 60 pending.

This PR was written by Claude Code on behalf of @apiology.