pkg: externalize host default assets

[fpolica91]

Summary

• move the containerd and network defaults to share/lima/defaults
• load host resources from installed share directories with a source-tree fallback for tests and local builds
• package and validate the external resources in release artifacts
• keep driver-specific and guest-agent assets embedded

Why

This starts #5295 with two host-only assets that have isolated consumers. The remaining CI-data, autostart, driver, and guest-agent embeds are intentionally unchanged so maintainers can review the resource-loading and packaging approach before expanding the scope.

Testing

• go test ./...
• go test -trimpath ./pkg/usrlocal ./pkg/limayaml ./pkg/networks
• make golangci-lint
• make yamllint
• make ls-lint
• make limactl
• Linux and Windows cross-builds
• CLI runtime smoke tests for network and containerd defaults
• positive and negative artifact validation fixtures

Related to #5295.