Make filesystem permission paths generic

[anp-oai]

Intent

Allow shared filesystem permission types to carry either app-native absolute paths or exec-server path URIs without changing existing app-side defaults.

Implementation

• Make the filesystem path containment hierarchy generic, defaulting to AbsolutePathBuf.
• Add canonical App* and Exec* aliases and use PathUri throughout exec-server filesystem protocol and trait boundaries.
• Convert between app and exec sandbox contexts only at host sandbox boundaries.