EPIC: Dead Code Cleanup and Dependency Hygiene

[bug-ops]

Problem

• 5 items suppressed with #[allow(dead_code)] — pre-positioned code for future features
• 3 potentially unused dependencies (async-trait, thiserror in channels, chrono in core)
• Feature combination CI matrix only tests default + full

Goal

Remove dead code, clean up unused dependencies, and improve feature coverage in CI.

Audit Reference

.local/audit/004-dead-code-report.md

Key Issues

• DEAD-01..05: Remove or gate 5 dead code items
• DEAD-DEP-01: Remove async-trait from root Cargo.toml
• DEAD-DEP-02: Verify thiserror usage in zeph-channels
• DEAD-DEP-03: Verify chrono usage in zeph-core
• DEAD-FEAT-01: Expand CI feature matrix

Acceptance Criteria

• Zero #[allow(dead_code)] annotations for non-test code
• No unused dependencies reported by cargo-machete (excluding false positives)
• CI matrix tests at least 4 feature combinations
• All tests pass