Why

Executor-owned paths must remain portable while the orchestrator validates them. Converting a Windows or remote path into the orchestrator host's native Path just to check containment applies the wrong filesystem rules.

This PR adds the lexical containment primitive needed by the URI-native plugin stack.

What changed

• Add PathUri::starts_with for lexical equality and containment.
• Compare URI authorities and complete path segments, so plugin-other is not treated as a child of plugin.
• Fail closed for encoded native path separators.
• Treat opaque fallback URIs as containing only themselves.
• Cover POSIX, Windows drive, UNC, authority, segment-boundary, encoded-separator, and opaque URI cases.

file:///C:/plugins/foo/assets/icon.svg
  is below file:///C:/plugins/foo

file:///C:/plugins/foo2/icon.svg
  is not below file:///C:/plugins/foo

Stack

1. This PR — add lexical PathUri containment.
2. Decouple plugin manifest path resolution #29620 — make manifest path resolution URI-native through one shared parser.
3. Make selected plugin roots URI-native #28918 — keep selected plugin roots and resources URI-native.
4. Load executor skills without host path conversion #29626 — load executor skills without host path conversion.
5. Keep executor plugin MCP paths URI-native #29628 — resolve executor MCP working directories without host path conversion.