miner: recover veblop producer stall when a build wedges with no pending task

[lucca30]

Problem

While stress-testing the veblop block-production path, a producer can enter a permanent stall: it stops producing/importing at a fixed height and recovers only after a process restart. Symptoms are Possible producer stall: veblop fallback skipping while chain is stale logged repeatedly with a static currentBlock and pendingTasksCount=0 — the producer's own stall detector fires, but the fallback never actually recovers.

Root cause

newWorkLoop's veblop stall-fallback skipped recovery whenever pendingWorkBlock == currentBlock+1, treating that value as proof the next block was being built. But commitWork only clears pendingWorkBlock via a defer ...Store(0) that runs on return. If a build blocks inside commitWork and never returns, pendingWorkBlock stays pinned at currentBlock+1 with no sealing task (pendingTasks empty). In that state the fallback short-circuited forever — the producer never resubmitted work, and the only escape was a restart.

(A build blocking inside commitWork is a separate concern being investigated independently. This PR addresses the recovery path so that such a hang is no longer permanent.)

Fix

• The fallback now treats pendingWorkBlock == next as "work in flight" only when a sealing task actually backs it. When pendingWorkBlock == next but no task exists, it distinguishes a wedged build from a slow-but-live one by how long the build has been outstanding (time since submit), not by block-timestamp age (chainAge is meaningless when the head carries an old timestamp, e.g. genesis). Recovery fires only past a 3×-blocktime threshold, so a live build is never interrupted.
• The fallback re-commit uses a non-blocking send, so newWorkLoop cannot itself wedge on the unbuffered newWorkCh when mainLoop is the blocked goroutine. It keeps emitting the stall warning and retrying every tick, and auto-recovers the moment mainLoop is unblocked.
• The decision is extracted into a pure decideVeblopFallback function so it can be unit-tested without driving the whole loop.

Testing

• New table-driven unit test TestDecideVeblopFallback covering: the wedged-build case (the bug), the genesis in-progress-build case (regression guard against interrupting a live build), task-in-flight, the no-work-claimed paths, and the wedge threshold boundaries.
• Verified as a true regression test: the suite goes red against the pre-fix logic (the wedged-build cases return "skip" instead of "recommit") and green against this fix.
• Full miner/ package suite passes (this caught an over-aggressive first attempt that used chainAge and interrupted live builds at genesis).

Scope / follow-ups

This restores producer-loop liveness and auto-recovery once the underlying build unblocks. It does not by itself resolve a hard mainLoop deadlock; that underlying build-blocking concern is tracked separately.

[claude]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.47%. Comparing base (5df879f) to head (1236875).

Files with missing lines	Patch %	Lines
miner/worker.go	86.36%	10 Missing and 2 partials ⚠️

❌ Your patch check has failed because the patch coverage (86.36%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2287      +/-   ##
===========================================
+ Coverage    53.44%   53.47%   +0.02%     
===========================================
  Files          896      896              
  Lines       159770   159840      +70     
===========================================
+ Hits         85397    85479      +82     
+ Misses       69054    69036      -18     
- Partials      5319     5325       +6     

Files with missing lines	Coverage Δ
miner/worker.go	77.93% <86.36%> (-0.08%)	⬇️

... and 24 files with indirect coverage changes

Files with missing lines	Coverage Δ
miner/worker.go	77.93% <86.36%> (-0.08%)	⬇️

... and 24 files with indirect coverage changes

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