Add retained-KV multi-turn chat to the Mac app

[drumih]

Summary

• retain one Mac app conversation and reuse its KV cache across text and image turns
• guard run identity, service epoch, and KV lineage; fail closed when recovery or context accounting is uncertain
• add New Chat, transcript copy controls, context reporting, and correct empty-state behavior
• ship the Mac app as version 0.7.0

Set TURBO_FIELDFARE_APP_CONVERSATION=0 to restore the single-prompt path.

Review fixes

• ignore terminal callbacks from older generation streams
• report failed KV rewind and stop-string cleanup immediately
• invalidate dead decode-service connections and release terminated routers
• carry exact computed-prefill and committed-KV diagnostics
• disable image attachments when the committed KV position is unknown
• validate context values before image-budget arithmetic

Verification

• Scripts/test.sh: 1,347 tests in 206 suites passed
• swift build -c release: passed
• Periphery 3.2.0: 115 existing findings, no new finding
• Markdown links, tracked symlinks, app version, and staged diff checks: passed
• tracked public/ and this branch: byte-identical
• M2 8 GB acceptance for the original feature passed five-turn text and image conversations, Stop/Continue, context refusal, lifecycle invalidation, transcript menus, and New Chat cleanup
• follow-up recovery paths are pinned by unit and pipe-backed integration tests