docs(stage6-7): add implementation strategy and roadmap documentation

- Add Stage 6: Implementation strategy (build, test, observability, release)
- Add Stage 7: Roadmap and task catalog with release planning
- Update spec index with new reading orders and stage summaries
- Update status board for Stage 6-7 completion
- Fix README image path for vitepress-docs
- Update mutex-queue ADR with additional context
- Bump version to 1.2.0

Author: wuchuheng

README.md

@@ -258,7 +258,7 @@ await db.query("SELECT * FROM notes WHERE id = ?", [1]);
 
 The console output highlights SQL keywords and shows how long each statement took (click to preview):
 
-[![Debug console output](docs/assets/debug.png)](docs/assets/debug.png)
+[![Debug console output](vitepress-docs/assets/debug.png)](docs/assets/debug.png)
 
 #### Transactions
 

docs/00-control/00-spec.md

@@ -274,7 +274,7 @@ await db.exec("VALID SQL"); // Succeeds (queue not broken)
 
 ## Navigation
 
-**Previous ADR**: [ADR-0002: OPFS Storage](./0002-opfs-persistent-storage.md) - File-based persistence
+**Previous ADR**: [ADR-0002: OPFS Storage](./0004-release-versioning-system.md) - File-based persistence
 
 **Next ADR**: [ADR-0004: Release Versioning](./0004-release-versioning-system.md) - Migration management