refactor(cluster): bring the calvin dispatch/scheduler and error files back under the 500-line limit

[EnRaiha]

Summary

Three files this branch touches exceed, or grow past, the house 500-line
non-test limit, so the local preflight (check C5) fails and was bypassed for the
fix push:

File	base	head
nodedb/src/control/cluster/calvin/scheduler/driver/core/dispatch.rs	504	542
nodedb/src/control/cluster/calvin/scheduler/driver/core/scheduler.rs	436	506
nodedb/src/error/types.rs	589	601

Work

• extract the busy helpers (busy_backoff, note_dispatch_busy,
  note_dispatch_busy_shared) from scheduler.rs into a small core/busy.rs;
• extract a self-contained piece of dispatch.rs (for example the active
  dispatch path) into its own module and thin the two dispatch sites;
• move any impl Error helper block out of error/types.rs into a sibling
  module (the Error enum itself cannot be split);
• target: every changed file at or under 500 non-test lines, preflight C5 green.

[EnRaiha]

Resolved on fix/calvin-backpressure (PR #355), pushed preflight-clean without a bypass:

File	base	before	now
core/dispatch.rs	504	543	387
core/scheduler.rs	436	508	459
nodedb/src/error/types.rs	589	601	586

• busy helpers (busy_backoff, note_dispatch_busy, note_dispatch_busy_shared) moved to core/busy.rs; the active dispatch path moved to core/active_dispatch.rs; the static dispatch site now calls note_dispatch_busy instead of duplicating the backoff block.
• error/types.rs has no impl Error block to move — the file is the enum plus the Result alias — and the enum cannot be split without changing the public API, so it cannot reach 500 non-test lines. It is back below its base count, which is the C5 rule for a pre-existing over-limit file: no growth.
• Evidence: preflight pass · Calvin determinism gate clean · cargo check -p nodedb and clippy clean · 217 calvin lib tests + busy_backoff_is_bounded_and_jittered pass.