Fix gamebox package loading on DOSBox Staging 0.78.1

[dtseto]

Summary

• mark a session ready to accept URLs after DOSBox reaches its initial or idle shell
• restore the previously launched program for both absolute and relative saved paths

Context

This is a focused backport of the two host-side BXSession fixes from branch-test-791b onto the DOSBox Staging 0.78.1 integration merged in #80. It intentionally excludes all 0.79/0.79.1 integration changes and the later integration-test target.

Validation

• based directly on the Branch test 78.1 #80 head (5ca68dd9)
• resulting diff is limited to Boxer/BXSession.m (5 additions, 1 deletion)
• git diff --check passes
• full isolated Xcode build was unavailable because the temporary worktree does not have its submodules initialized and the sandbox denied Xcode package-cache writes