feat(pi): add opt-in simple structural session policy

[rs545837]

Summary

• Package the tested simple session adapter as an opt-in policy; leave the default protocol unchanged.
• Include batched exact reads, acknowledged context reuse, scoped edit composition and stale-snapshot guards.
• Document setup, validation limits and the single-pair PlantUML result without claiming universal speedups.

Validation

• 52 policy tests passed with SEM 0.25.0.
• 13 Python validation-helper tests passed.
• 11 existing Weave and transaction integration tests passed.
• Typecheck passed.

The new policy remains experimental and opt-in.

[inspect-review]

inspect review

Triage: 258 entities analyzed | 0 critical, 0 high, 4 medium, 254 low
Verdict: standard_review

Findings (0)

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Reviewed by inspect | Entity-level triage found 0 high-risk changes

[inspect-review]

inspect review

Triage: 258 entities analyzed | 0 critical, 0 high, 4 medium, 254 low
Verdict: standard_review

Findings (0)

---

Reviewed by inspect | Entity-level triage found 0 high-risk changes