fix(consensus): count an unservable header as a nack

[krishvishal]

A view change can reach a state where an op is neither recoverable from the DoViewChange messages in hand nor provably uncommitted. merge_dvc_quorum answers AwaitingRepair and waits for every replica to report, so a permanently crashed replica leaves two healthy survivors unable to elect a primary. The view number keeps climbing while log_view stays put, and client requests retry forever.

The op was decidable from the quorum already present. A sender that holds the header but cannot serve the body counted as neither a copy nor a nack. A prepare is journaled before it is acked and nothing compacts an op above the commit point, so a missing body proves that sender never acked it, which is what a nack asserts.

tally_op now derives a nack from that sender. The quorum is unchanged at replica_count - quorum_replication + 1, so replicas provably outside the ack set still leave fewer than a replication quorum inside it, and a sender that can serve the body is untouched. The derived nack stops at the sender's own commit point, where a missing body means compaction rather than absence.

Two unit tests now assert truncation where they asserted a stall, one pins the boundary at a single servable copy, and the undecidable case moves to abstention. A simulator test replays the seed that found this: without the change the cluster reaches view 103 against log_view 3 with both survivors caught up, and one request retries 266 times unanswered.

[codecov]

Codecov Report

❌ Patch coverage is 93.80952% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.18%. Comparing base (f87ef77) to head (afcd4a9).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/consensus/src/dvc_merge.rs	86.84%	5 Missing ⚠️
core/partitions/src/iggy_partition.rs	63.63%	0 Missing and 4 partials ⚠️
core/consensus/src/plane_helpers.rs	98.86%	0 Missing and 1 partial ⚠️
core/metadata/src/impls/metadata.rs	80.00%	0 Missing and 1 partial ⚠️
core/shard/src/lib.rs	66.66%	1 Missing ⚠️
core/simulator/src/lib.rs	97.95%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #4109       +/-   ##
=============================================
- Coverage     86.38%   72.18%   -14.21%     
+ Complexity     1455     1454        -1     
=============================================
  Files          1259     1257        -2     
  Lines        205513   183289    -22224     
  Branches     170714   148511    -22203     
=============================================
- Hits         177542   132301    -45241     
- Misses        23541    46631    +23090     
+ Partials       4430     4357       -73     

Components	Coverage Δ
Rust Core	69.67% <93.80%> (-17.67%)	⬇️
Java SDK	67.57% <ø> (-0.02%)	⬇️
C# SDK	76.97% <ø> (ø)
Python SDK	91.34% <ø> (ø)
PHP SDK	85.65% <ø> (-0.01%)	⬇️
Node SDK	96.15% <ø> (-0.03%)	⬇️
Go SDK	69.43% <ø> (+0.03%)	⬆️

Files with missing lines	Coverage Δ
core/consensus/src/impls.rs	93.06% <100.00%> (+0.13%)	⬆️
core/consensus/src/plane_helpers.rs	95.30% <98.86%> (-0.13%)	⬇️
core/metadata/src/impls/metadata.rs	88.88% <80.00%> (-0.31%)	⬇️
core/shard/src/lib.rs	85.13% <66.66%> (+0.37%)	⬆️
core/simulator/src/lib.rs	96.35% <97.95%> (+0.01%)	⬆️
core/partitions/src/iggy_partition.rs	92.30% <63.63%> (-0.04%)	⬇️
core/consensus/src/dvc_merge.rs	93.33% <86.84%> (-0.29%)	⬇️

... and 325 files with indirect coverage changes

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