Reconciled stale assurance and test docs

[shrec]

What

• Updated SECURITY.md to reflect the current active release line and completed assurance backlog items
• Refreshed docs/TEST_MATRIX.md to the current 182-target validation surface and added the missing exploit/ABI targets
• Reframed docs/CROSS_PLATFORM_TEST_MATRIX.md as the 41-test portability subset over the larger validation surface, with updated counts and module totals
• scripts/validate_assurance.py --json now returns total_issues: 0 again

Numbers

• Validation surface: 162 -> 182 documented CTest targets
• Assurance validator: 13 -> 0 issues
• Unified audit module count in docs: 49 -> 70

How to get it

• Branch: dev
• Commit: 4bb344c4