Follow Antora cross-file cross-references

[bbatsov]

Phase 2 of the Antora/xref work, building on the section auto-ids from #80.

In a file inside an Antora component (a tree with an antora.yml), following an xref: that targets another page now actually goes there. xref:basics/install.adoc[], xref:other.adoc#a-section[], and module:page.adoc all resolve to the right file under the target module's pages/ directory, and the #fragment jumps to the matching section (reusing the auto-id resolver). It works from C-c C-o / M-. and a mouse click, pushes the xref marker stack so M-, returns, and stays out of the way outside Antora (a plain .adoc xref elsewhere falls through to the in-buffer handling).

Resolution is deliberately scoped to the current component - version@ and component:module: cross-component targets need Antora's site catalog, which we don't have, so they're left alone. I verified it against my real cider/doc and rubocop/docs layouts.

Still to come in later PRs: completion of xref page targets + #fragments, and project-wide xref-find-references; then paste-image.

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