XY-1154: Implement Context Pack v1, automatic routing, recall engine, and Recall Debug traces

[yvette-carlisle]

Summary

• Implement Context Pack v1 as a read-time scoped view over recall/debug layers.
• Add automatic routing, typed layer selectors, activation trace output, and pack item eligibility safeguards.
• Expose the pack builder through HTTP and MCP without public debug overrides.
• Document the Context Pack v1 contract and version registry entries.

Validation

• cargo test -p elf-service context_pack -- --nocapture
• cargo make fmt
• cargo make lint-fix
• cargo make checks

Review

• Independent fresh-context review checkpoint is clean for HEAD e45c660.