refactor: delete unused precompute coordinator and series buffer

[zzylol]

Why

Step toward DAG-only precompute execution: remove precompute modules that no
production path uses, so the remaining engine code reflects what actually runs.

What

Delete three modules (1.8k lines) that are referenced only by each other and
by precompute_engine/mod.rs:

• multisource_coordinator.rs (1104 lines)
• coordination_checkpoint.rs (548 lines)
• series_buffer.rs (163 lines)

precompute_engine_design_doc.md drops the SeriesBuffer section (§3.5,
later sections renumbered), its SeriesState field and the module-table and
unit-test mentions.

Before this PR

$ git grep -l 'multisource_coordinator\|coordination_checkpoint\|SeriesBuffer' -- '*.rs'
data_plane/src/precompute_engine/coordination_checkpoint.rs
data_plane/src/precompute_engine/mod.rs
data_plane/src/precompute_engine/multisource_coordinator.rs
data_plane/src/precompute_engine/series_buffer.rs

After this PR

No references remain. No behavior change.

Not in scope

• Raw passthrough (pass_raw_samples): default-off but a documented benchmark
  mode, so removing it is a separate decision.
• Deprecated aliases in hot_reload_config.rs and test-only legacy
  accumulator paths: they overlap Bind Planner physical candidates without Backend operator lowering #788 and follow it.

Validation

cargo clippy --workspace --all-targets --locked -- -D warnings and
cargo fmt --check pass; cargo test --workspace --locked --lib: 1449 passed.

🤖 Generated with Claude Code