Cache-stable safeDeltaInsertIndex

Chose immutable knowledge-delta blocks at a safe message-tail boundary over rewriting system prefixes or shifting prior deltas because prefix movement breaks caches. safeDeltaInsertIndex and compression re-anchoring must walk backward and place deltas before assistant tool calls, never between tool_use and the immediately following user tool_result, including parallel tools and tail re-anchoring; preserve surfaced mutation history. Keep at most two cache breakpoints: one interior and the tail.