fix: don't close signal channel in walker goroutines to prevent panic

[Yanhu007]

Fixes #37

Problem

AncestorsWalker and DescendantsWalker close both ids and signal channels when the walk completes. If the consumer stops at the last vertex and then sends on the signal channel, it panics:

panic: send on closed channel

Root Cause

The goroutine closes signal after the walk finishes, but the consumer may try to send on it after receiving the last vertex:

// goroutine
close(ids)
close(signal) // ← closes signal

// consumer
signal <- true // ← panic: send on closed channel

Fix

Remove close(signal) from both walker goroutines. The signal channel is a send-only channel from the consumer's perspective — the goroutine should not close it. The consumer signals the goroutine to stop; the goroutine signals the consumer by closing ids.

All existing tests pass.

[sebogh]

Thanks @Yanhu007