[Book][v4.30.0] Add Stacks Project Chapters 4-7

[imathwy]

Summary

• Add the migrated Stacks Project Chap04--Chap07 source closure as StacksProject_Chapters_2026.
• Register it as a ReasBook library while preserving the stacks_project.* namespace.
• Preserve Stacks/Kerodon tags in stacks-tags.json; map the RC-only Cofinality import to stable v4.30.0 Regular.

Verification

• book.yml passes the ReasBook schema and toolchain checks.
• git diff --check passes.
• Representative module: lake -R build stacks_project.Chap04.Definition_4_20_1.

Known limitation

The source snapshot is from v4.30.0-rc1. The full Chap04--Chap07 aggregate is intentionally not a merge gate yet: Chap07/Lemma_7_17_10 still has Lean 4.30 API-porting type errors. No proofs were replaced with sorry to hide those errors; see STACKS_PR_BUILD_STATUS.md.