Timeline-blind end-of-WAL marker can poison WAL restore after timeline switch

`pkg/restorer.WALRestorer` tracks end-of-WAL state with a single global spool marker named `end-of-wal-stream`. That marker is not scoped to the WAL timeline. This is unsafe when a restore flow uses parallel prefetch and PostgreSQL later requests WAL across a timeline switch. A prefetch miss on timeline N can set the global marker, and the next restore request on timeline N+1 can consume that marker before checking archive/spool state for the requested child-timeline segment.

A full repro is available here: https://github.com/ardentperf/bug-repro-wal-restore

The problem is not Barman core's normal behavior of returning failure when a requested WAL is unavailable. That is expected and is how PostgreSQL can move from archive restore to streaming, but it needs to be timeline scoped.