## Why
Plugin installation skips symlinks while copying a plugin into the cache. A
symlinked manifest could therefore be used during source validation but omitted
from the staged copy, allowing a lower-precedence manifest to take its place.
## What changed
- Require discoverable manifests and their parent directories to be regular
files and directories, rejecting symlinks and other non-regular entries at a
higher-precedence manifest path.
- Verify that staging preserves the selected manifest path and contents before
activating the cached plugin.
- Preserve generated fallback manifests by injecting and validating them in the
staged plugin.
## Testing
Add coverage for symlinked manifest files and directories, precedence changes,
and fallback-manifest staging.
GitOrigin-RevId: 1b69c1e75cdfecb8cab4070a32aa9739833a70de
e7c0e8eb9f