ci: select native docs runners before matrix allocation

[injaneity]

Before

The docs workflow allocated all four native matrix jobs before determining which library changed. Each runner installed documentation dependencies first, and any matching driver change selected all three driver platforms. An additional job aggregated the results.

After

Four workflow-level path filters select native checks before runners are allocated:

Changed inputs	Native docs jobs
Linux, macOS, or Windows adapter	Corresponding driver platform
Shared driver code, build inputs, or CLI reference	All three driver platforms
Platform-specific MCP reference	Owning driver platform
Lume source, package inputs, or generated references	Lume on macOS
Shared generator, dependency, or reusable-workflow inputs	All affected platforms
Unrelated or curated documentation	None

The four entry workflows call one reusable workflow containing the native installation, documentation extraction, ownership tests, and policy checks. There is no selection runner, dynamic matrix, or aggregate job. Generator code and configuration are unchanged.

The removed Check Documentation Sync aggregate is not required by the current effective main-branch rules. Branch protection is unchanged. Native workflow path filtering remains subject to GitHub’s diff limits, including the 300-file path-filter limit.

Refs #3722.