docs: update all docs for sessions, solver, approvals, and recent features

- STATUS.md: migration count 18→23, endpoint count 86→~140, add approval
  workflows, solver service, workstations, edit sessions, SSE targeted
  delivery rows, update test file count 9→31, add migrations 019-023
- MODULES.md: add solver and sessions to registry, dependencies, endpoint
  mappings (sections 3.11, 3.12), discovery response, admin settings,
  config YAML, and future considerations
- CONFIGURATION.md: add Approval Workflows, Solver, and Modules config
  sections, add SILO_SOLVER_DEFAULT env var
- ROADMAP.md: mark Job Queue Complete (Tier 0), Audit Trail Complete
  (Tier 1), Approval/ECO Complete (Tier 4), update Workflow Engine tasks,
  add Recently Completed section, update counts, resolve job queue question
- GAP_ANALYSIS.md: mark approval workflow Implemented, locking Partial,
  update workflow comparison (C.2), update check-in/check-out to Partial,
  task scheduler to Full, update endpoint counts, rewrite Appendix A
- INSTALL.md: add MODULES.md, WORKERS.md, SOLVER.md to Further Reading
- WORKERS.md: status Draft→Implemented
- SOLVER.md: add spec doc, mark Phase 3b as complete

docs/INSTALL.md

@@ -491,4 +491,7 @@ After a successful installation:
 | [SPECIFICATION.md](SPECIFICATION.md) | Full design specification and API reference |
 | [STATUS.md](STATUS.md) | Implementation status |
 | [GAP_ANALYSIS.md](GAP_ANALYSIS.md) | Gap analysis and revision control roadmap |
+| [MODULES.md](MODULES.md) | Module system specification |
+| [WORKERS.md](WORKERS.md) | Job queue and runner system |
+| [SOLVER.md](SOLVER.md) | Assembly solver service |
 | [COMPONENT_AUDIT.md](COMPONENT_AUDIT.md) | Component audit tool design |