Move AnyChannel and dispatch_channel! to zeph-channels

Part of #391

## Problem

`AnyChannel` enum and `dispatch_channel!` macro live in `src/main.rs` (lines 51-122). Every new channel implementation requires updating these in the binary crate.

## Proposed Solution

Move `AnyChannel` and the dispatch macro to `zeph-channels` crate with feature-gated variants.

**Priority:** P1 | **Effort:** Small