[storage/qmdb] Coordinate state sync finalization

[clabby]

Overview

Adds explicit finish coordination to QMDB's state sync engine and exposes reached-target notifications so external orchestration can coordinate multiple databases syncing towards a unified set of targets.

Motivation

In #3381, we store the state root and range of each managed database in each block that is formed. When state syncing multiple databases, we must finish state sync for each DB at the same point. To synchronize these tasks, we need:

1. A way for each engine to signal “I reached target X”.
2. A way to hold completion until the orchestrator says “finish now”.

With this, we can hold off the completion of state sync for a set of databases until the full set has converged on the set of targets.

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	commonware-mcp	79a8184	Mar 23 2026, 05:43 PM

[danlaine]

nit: I was inconsistent with _rx/_tx vs _receiver/_sender -- should we take this opportunity to make them all _rx/_tx? i.e. change the field names here to match those in the config

[clabby]

Unified the names on _rx / _tx.

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Latest commit:	79a8184
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[danlaine]

Would you mind adding tests for:

1. Finish signal arrives before target reached
2. Caller drops finish_tx without having sent a signal
3. Caller drops reached_target_rx without having received a signal

Otherwise lgtm

The base branch was changed.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.65%. Comparing base (921c288) to head (79a8184).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
storage/src/qmdb/sync/engine.rs	92.22%	5 Missing and 2 partials ⚠️

@@            Coverage Diff             @@
##             main    #3439      +/-   ##
==========================================
- Coverage   95.65%   95.65%   -0.01%     
==========================================
  Files         421      421              
  Lines      148789   148877      +88     
  Branches     3523     3530       +7     
==========================================
+ Hits       142320   142401      +81     
- Misses       5335     5340       +5     
- Partials     1134     1136       +2     

Files with missing lines	Coverage Δ
storage/src/qmdb/immutable/sync.rs	98.45% <100.00%> (+0.05%)	⬆️
storage/src/qmdb/sync/error.rs	0.00% <ø> (ø)
storage/src/qmdb/sync/engine.rs	89.60% <92.22%> (+1.25%)	⬆️

... and 7 files with indirect coverage changes

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