fix: close execution ingress before shutdown synchronization

[bmuddha]

What changed

Closes #175

Engine now sends a terminal sequencer message for shutdown. The sequencer closes ingress and drains scheduled execution, then remains paused while Engine dumps external-pacing state and synchronizes storage. It does not resume to process submissions queued behind the terminal marker.

Impact

Waiting sends fail after ingress closes. Work queued behind the marker is dropped when the sequencer exits, so its execution waiters receive channel errors; queueing acknowledgment is not an execution guarantee during shutdown. Existing submission APIs and From<ExecutionRequest> remain unchanged.

Reviewer notes

Ordinary snapshot barriers and shutdown-tier ordering are unchanged. No ledger behavior changes.

[coderabbitai]

📝 Walkthrough

Shutdown now uses a terminal sequencer message to close execution ingress and drain scheduled work before storage synchronization. Submissions queued behind the marker are dropped when the sequencer exits. A shared pause helper submits barrier-based messages and is used by Engine shutdown, public barriers, and pacemaker checkpoint handling.

Priority: ➖ Normal

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: 🔵 Low · up to 9f710

The new shutdown marker correctly closes execution ingress and drains scheduled work. However, if the shutdown signal reaches an idle sequencer before the marker is sent, the sequencer exits early and shutdown can skip the final storage sync. This race already existed before the change, so the change does not make shutdown less safe. It is worth fixing, ideally as part of this shutdown-ordering work.

Pre-merge checks: 5

Passed checks: 5 passed

Check name	Status	Explanation
Description check		The description clearly explains the shutdown ingress, sequencing, queued work, and compatibility changes covered by the pull request.
Title check		The title accurately and concisely describes the main change: closing execution ingress before shutdown synchronization.
Linked Issues check		Issue #175 requires shutdown to close execution ingress, drain scheduled work, and prevent work queued behind the terminal boundary from executing before storage closes. Engine::shutdown sets `termi…
Out of Scope Changes check		The changes stay within Issue #175. The Engine::pause helper and the pacemaker conversion preserve barrier submission while centralizing the shutdown and barrier flow. The runtime, processor, engine…
Docstring Coverage		Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 4 files. (3 skipped: 3 …

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @processor/src/sequencer/mod.rs:
- Line 177: Update the sequencer shutdown branch around
self.shutdown.signalled() so a shutdown signal cannot make an idle sequencer
exit before it processes the terminal marker and acknowledges Engine::pause.
Coordinate the signal and marker handling to keep the sequencer alive through
marker processing and storage synchronization.

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📥 Commits

Reviewing files that changed from the base of the PR and between 698d037 and 9f71041.

📒 Files selected for processing: 7

• README.md
• engine/README.md
• engine/src/lib.rs
• engine/src/pacemaker.rs
• nucleus/src/runtime.rs
• processor/README.md
• processor/src/sequencer/mod.rs

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