fix(shard): drive metadata repair and the commit walk from the tick

[numinnex]

A metadata backup that fell behind had no way back on its own, the same starvation the partition plane just had. A follower advances commit_max from every prepare header in replicate_preflight, before the gap check drops the prepare, so the commit heartbeat lands as Accepted rather than Advanced and the arming site inside that branch never fires. Under sustained metadata traffic the gap wedged until an unrelated view change. The same starvation left a backup holding resident committed ops that nothing re-drove, since the follower walk runs at the tail of an accepted prepare and the gap check returns before it.
▎
▎ The metadata tick now evaluates the same two level-triggered checks the partition driver uses, off one probe. A hole below commit_max arms the existing repair against the primary, debounced on the repair retry interval. Resident committed ops re-drive the walk, follower only, because a backup's walk ships no wire replies while a stranded primary is resume_stranded_commits' job. A gap below the serving peer's retention floor needs no special handling: the serve path answers RangeEvicted and the existing conversion arms a state transfer.

[numinnex]

Merge after #4006

[codecov]

Codecov Report

❌ Patch coverage is 94.63972% with 61 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.94%. Comparing base (c8ef3b9) to head (d5c8ba1).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
core/simulator/src/lib.rs	92.37%	33 Missing and 13 partials ⚠️
core/shard/src/lib.rs	96.24%	15 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4008      +/-   ##
============================================
- Coverage     84.91%   83.94%   -0.98%     
  Complexity     1405     1405              
============================================
  Files          1224     1225       +1     
  Lines        179327   180931    +1604     
  Branches     145615   147219    +1604     
============================================
- Hits         152273   151879     -394     
- Misses        23026    25083    +2057     
+ Partials       4028     3969      -59     

Components	Coverage Δ
Rust Core	85.90% <94.63%> (+0.10%)	⬆️
Java SDK	67.35% <ø> (ø)
C# SDK	52.64% <ø> (-22.76%)	⬇️
Python SDK	90.06% <ø> (ø)
PHP SDK	85.65% <ø> (ø)
Node SDK	96.18% <ø> (+0.04%)	⬆️
Go SDK	69.36% <ø> (+0.03%)	⬆️

Files with missing lines	Coverage Δ
core/metadata/src/impls/metadata.rs	88.42% <100.00%> (+0.36%)	⬆️
core/partitions/src/iggy_partition.rs	89.74% <100.00%> (+0.14%)	⬆️
core/server/src/partition_reconciler.rs	97.86% <100.00%> (-0.10%)	⬇️
core/shard/src/metrics.rs	87.74% <100.00%> (+1.18%)	⬆️
core/shard/src/router.rs	78.58% <100.00%> (+0.31%)	⬆️
core/shard/src/lib.rs	83.57% <96.24%> (+1.38%)	⬆️
core/simulator/src/lib.rs	96.22% <92.37%> (-0.73%)	⬇️

... and 108 files with indirect coverage changes

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