Design path overcounting shared uncertainty fixture

[InfoSecHack]

Summary

Adds a docs-only fixture design for the local-only Path Overcounting and Shared Uncertainty demo.

The design defines:

• target demo behavior and exact intended counts;
• modeled path categories: validated, blocked, precondition_only, inconclusive;
• shared uncertainty classes, including the primary 8-path unresolved evidence group;
• proposed future fixture files and generated /tmp outputs;
• safety boundaries and test plan;
• exactly one next slice: build local synthetic fixture.

Scope

Docs/design only. No implementation changes, benchmark semantic changes, live AWS, new reasoners, new evidence claims, generated outputs, or release changes.

Validation

• ./scripts/check.sh
• ./scripts/test_fast.sh
• git diff --check