GitHub validation isolation

This records the initial isolation step. Package impact extends its conservative fallback with the actual workspace consumption graph.

Project and generator workflows select impact on every pull request and main push. Only a complete nonempty set of paths under website/ and docs/ skips package and generator execution. Root formatting, code lint, Markdown lint, spelling and documentation validation still run. Documentation feeds website pages, so website validation remains necessary. Shared, mixed and unknown paths retain broad checks. Renames include both paths; additions and deletions participate in the same selection. Missing event revisions fall back to broad work; an actual Git diff error fails selection.

No existing required context was renamed or removed. Each unaffected context starts and reports success explicitly. Each context checks selection and all its prerequisites; failed, cancelled, skipped or missing prerequisite results cannot become successful no-ops. Selection itself appears as Select validation impact in each workflow. There is no repository-settings migration in this change. The 27 required contexts recorded in the baseline remain the protection contract.

To roll back, revert the selection commit; the original broad execution and context names return together. Any future consolidation of required contexts needs a separate approved protection migration. Local workflow contracts validate selection and guard behavior; live GitHub runs are still needed to establish runner and protection behavior.

Tool validation and Generator validation summarize every job in their own workflow, including impact selection. They fail when any prerequisite fails or is cancelled or skipped. These new summaries are additional checks; adopting them as required contexts must be separately approved before removing existing protections.