feat: add multi-party memory isolation

[EvanProgramming]

Summary

• add speaker-, audience-, channel-, visibility-, and authorization-scoped memory claims
• preserve attributed beliefs and per-speaker supersession while abstaining on ambiguity
• bind private API access to the authenticated request actor and emit attribution receipts
• add frozen multi-party leakage, update, audience, and ambiguity benchmark cases

Schema and API

• extend claim metadata with claim_type, speaker, audiences, channel, and visibility
• extend GET /api/v2/memory and claim inspection filters
• add POST /api/v2/memory/claims for typed claim creation
• include memory_receipt in chat and streaming responses

Security impact

• private facts require explicit owner authority
• client-supplied speaker labels do not grant private-memory access
• private values are redacted from claim audit events and excluded from unscoped recall
• existing capability, profile, workspace, and external-skill boundaries are unchanged

Validation

• make check
• KYROZEN_DB_PATH=<temporary sqlite path> make test (58 tests)
• API smoke: health, scoped memory query, invalid actor rejection
• frozen benchmark replay: 5 cases, neutral replay emits no superiority claim
• git diff --check