Improve segmentation effective policy evidence

[DENGXUELIN]

/claim #1394

Summary

• Adds effective policy decision gates to segmentation so reviews do not pass a flow based only on policy-object presence.
• Requires policy-engine inventory, runtime identity, selector resolution, ordering/fallthrough, deny precedence, enforcement mode, and expected-vs-observed flow evidence.
• Adds skill-local JSON fixtures for a vulnerable manifest-only/shadowing case and a benign complete effective-decision matrix.

Validation

• git diff --cached --check
• git diff --check origin/main...HEAD
• Parsed both JSON fixtures with ConvertFrom-Json
• Verified Markdown fence balance
• Verified SEG-EFF-01 through SEG-EFF-08 markers
• Added-line sensitive-pattern scan
• git merge-tree --write-tree origin/main HEAD matched HEAD^{tree}