docs: add domain expansion roadmap entries and spec candidates (#178)
## Summary
Adds roadmap-level entries and qualified spec candidates for four missing high-value domain expansions, aligning product docs with already-discussed platform coverage direction.
### New Roadmap Entries (Mid-term)
- **Entra Role Governance** — identity administration posture, role definition/assignment visibility
- **SharePoint Tenant-Level Sharing Governance** — tenant-wide sharing/external access posture
- **Enterprise App / Service Principal Governance** — privileged permissions, expiring credentials, review workflows
- **Security Posture Signals** — Defender VM exposure, backup assurance, evidence inputs for reviews
### New Spec Candidates (Qualified)
| Candidate | Priority |
|-----------|----------|
| Enterprise App / Service Principal Governance | high |
| SharePoint Tenant-Level Sharing Governance | medium |
| Entra Role Governance | medium |
| Security Posture Signals Foundation | medium |
### What this does NOT change
- No strategy/domain-coverage doc changes
- No existing roadmap structure rewrite
- No existing candidate duplication
- No implementation specs or code changes
### Files modified
- `docs/product/roadmap.md`
- `docs/product/spec-candidates.md`
Co-authored-by: Ahmed Darrazi <ahmed.darrazi@live.de>
Reviewed-on: #178
ahmido
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