feat(manifest): add allow-listed per-netns sysctls primitive

Routing apps (a rootless VPN exit) need packet forwarding in their own
network namespace, but /proc/sys is read-only inside a rootless
container, so it can only be set at create time. Add `app.sysctls`,
allow-listed to net.ipv4.ip_forward / net.ipv6.conf.all.forwarding with
values "0"/"1", and rejected under host networking where it would change
the host. Rendered on all three create paths: podman CLI --sysctl, the
libpod spec `sysctl` map, and Quadlet `Sysctl=`. Absent by default and
not serialized when empty, so existing manifests and units are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/app-developer-guide.md

@@ -124,6 +124,7 @@ app:
 | `app.environment` | Static `KEY=value` environment entries |
 | `app.health_check` | HTTP or TCP health check settings |
 | `app.devices` | Explicit device paths |
+| `app.sysctls` | Namespaced packet-forwarding sysctls for the app's own network namespace (allow-listed; not with host networking) |
 | `app.metadata` | Catalog-facing presentation metadata such as icon, category, tier, repo/source, author, feature bullets, and [launch hints](#browser-iframe-and-companion-launch-modes) |
 | `app.interfaces.main` | Optional primary UI launch surface with `port`, `protocol`, and `path` |