Add Options.FSRoot to bound pom.xml parent resolution

[andrew]

pomXMLParser is the only parser that touches the filesystem: it follows <parent><relativePath> via pom.LocalFetcher so multi-module projects resolve ${project.version} and parent-defined properties. Server-side that means a pushed pom.xml can read arbitrary files relative to the process working directory.

This adds manifests.Options{FSRoot string} and threads it through a new core.FSRootParser interface that only pomXMLParser implements. With FSRoot empty (the default, and what existing Parse(filename, content) calls get), parsing is a pure function of content and the parent walk is skipped. With FSRoot set, parent lookup is jailed to that directory by pom.NewLocalFetcherFrom (git-pkgs/pom#10, released as v0.1.5).

Parse keeps its existing signature via a variadic opts ...Options so current callers compile unchanged; they lose multi-module parent resolution until they pass Options{FSRoot: repoRoot}.