refactor: delete unused precompute coordinator and series buffer (#790)

`multisource_coordinator`, `coordination_checkpoint` and `series_buffer`
have no callers outside themselves and `mod.rs`; production precompute
does not construct them. Remove them (1.8k lines) and their design-doc
sections.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: zzylol