Fix bridged startup and private Ninja test execution

[themartiano]

Fix two startup/build failures introduced in #198 and #201:

• Bridged startup now uses exec inside command substitution so the networking client remains a direct child of the launcher under macOS /bin/bash. This preserves the process chain required by the privileged daemon without weakening authorization.
• The libslirp Meson test step now uses --no-rebuild after the explicit private-Ninja build. Clean machines no longer need a globally installed Ninja to run those tests.

Validation:

• Added a regression that invokes the real qemu_network_start under /bin/bash with a native client stub that checks its parent PID. It fails before the fix and passes afterward.
• Executed the production Meson test command against a minimal project with the checksum-pinned Meson 1.9.0 and Ninja 1.13.0. With Ninja absent from PATH, the old command exits 127 and the fixed command passes.
• Full make test passed.

A complete QEMU runtime rebuild and privileged bridged-VM launch were not performed for this change.