New: five audience-targeted runnable demos + architecture docs

[cognis-digital]

We added five narrated, runnable demo scenarios under demos/, each written for a different FedRAMP audience and driving the real fedramplens API over bundled boundary fixtures, fully offline (exit 0):

1. 01_pm_authorization_readiness.py — Program Managers: a portfolio ATO-readiness gate with an escalation list.
2. 02_assessor_sarif_review.py — 3PAOs/assessors: findings as a SARIF 2.1.0 log for code-scanning.
3. 03_platform_engineer_boundary_map.py — platform engineers: Mermaid + Graphviz boundary map and unencrypted-crossing (SC-8) findings.
4. 04_isso_oscal_packages.py — ISSOs: generate and inspect the OSCAL SSP and POA&M.
5. 05_offline_control_enrichment.py — air-gapped ISSOs/assessors: resolve real NIST 800-53 rev5 control titles offline, with graceful degrade.

Run them all with PYTHONUTF8=1 python demos/run_all.py. New docs: docs/ARCHITECTURE.md (boundary -> OSCAL Mermaid flow) and docs/DEMOS.md (audience table). Feedback welcome.