docs(cua-driver): separate native MCP references from shared documentation

[injaneity]

Problem

Cua Driver documentation generation writes host-native MCP tool definitions to one shared reference. Linux/Windows regeneration overwrites macOS content, while docs CI checks only macOS. Shared CLI documentation is currently platform-independent.

Maintainer-selected scope

Prototype the Linux/macOS vertical slice: host-specific MCP output paths, shared MCP landing page, unchanged shared CLI reference, native drift lanes, navigation and contributor instructions. Reuse native dump-docs and the existing renderer; do not move runtime metadata.

Acceptance

• Linux generation updates only the Linux MCP page and shared CLI page; macOS behaves symmetrically.
• Neither generator overwrites shared guidance or the other platform page.
• Same-source shared CLI output matches across platforms.
• Repeat generation is stable; native drift checks, links, hygiene and docs build pass.

Landing blockers

This slice stays draft until Windows generation/validation, release-workflow aggregation, and old-link migration are complete. No runtime behavior changes.

Related: #3617 exposed the workflow problem; #3293 scoped docs validation but did not separate platform output.

[injaneity]

Implementation draft: #3620. Linux/macOS output routing is started; native evidence and the documented landing blockers will be tracked there.

[injaneity]

Scope simplified at maintainer request in #3620: retain the existing macOS mcp-tools.mdx URL and all headings; add only the Linux native page; reuse mcp-tool-notes.mdx for shared guidance. No new landing page or macOS rename. Local validation passes; revised native CI is pending. Windows and release-time aggregation remain draft landing blockers.