feat(server-ng): close the partition materialisation race at the owner

[krishvishal]

Problem

create_topic returns on metadata commit; the owning shard publishes the partition only after build_partition_fresh (mkdir, segment open, fallocate). A produce issued right after races materialisation, on every shard.

partition_reconciler carried a TODO for a materialisation barrier: stop non-owners seeding shards_table rows, have the owner push its row to peers, so row presence implies the partition exists.

Rejected. It turns a deterministic hash cache into a distributed readiness protocol whose deltas need ordering, retry and repair, and it gives up level-triggered repair (a pass re-derives every row from committed metadata, so a lost row is rewritten). The owner can decide all of it locally.

Approach

Resolve at the owner; shards_table stays a hint.

• router::route_typed falls back to calculate_shard_assignment on a missing row, so an early frame reaches the future owner.
• park_if_unmaterialised holds it until the matching InsertOwned, then re-queues onto that shard's inbox.
• serves_committed_incarnation refuses a namespace whose committed created_revision disagrees with the row epoch, so a mid-teardown request cannot be acked against an incarnation about to be erased.

Tombstoned namespace, park overflow, and a namespace the shard gave up building all reply with a retriable status. No silence, so no lockstep transport waits out its read timeout.

[codecov]

Codecov Report

❌ Patch coverage is 90.92382% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.77%. Comparing base (53a7ff9) to head (2c2610e).

Files with missing lines	Patch %	Lines
core/shard/src/lib.rs	76.02%	43 Missing and 4 partials ⚠️
core/server-ng/src/partition_reconciler.rs	98.96%	4 Missing ⚠️
core/shard/src/router.rs	42.85%	4 Missing ⚠️
core/shard/src/metrics.rs	94.73%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3786       +/-   ##
=============================================
- Coverage     75.58%   55.77%   -19.81%     
  Complexity      969      969               
=============================================
  Files          1317     1315        -2     
  Lines        155244   137476    -17768     
  Branches     128866   111178    -17688     
=============================================
- Hits         117339    76681    -40658     
- Misses        34361    57353    +22992     
+ Partials       3544     3442      -102     

Components	Coverage Δ
Rust Core	50.76% <90.92%> (-24.79%)	⬇️
Java SDK	62.71% <ø> (ø)
C# SDK	71.14% <ø> (-1.19%)	⬇️
Python SDK	92.27% <ø> (ø)
PHP SDK	84.52% <ø> (ø)
Node SDK	95.31% <ø> (+0.11%)	⬆️
Go SDK	43.08% <ø> (ø)

Files with missing lines	Coverage Δ
core/metadata/src/stm/stream.rs	62.17% <100.00%> (+0.19%)	⬆️
core/server-ng/src/dispatch.rs	40.00% <100.00%> (+0.19%)	⬆️
core/simulator/src/lib.rs	97.09% <100.00%> (+<0.01%)	⬆️
core/shard/src/metrics.rs	89.51% <94.73%> (+0.94%)	⬆️
core/server-ng/src/partition_reconciler.rs	91.42% <98.96%> (+4.09%)	⬆️
core/shard/src/router.rs	47.94% <42.85%> (+0.32%)	⬆️
core/shard/src/lib.rs	58.37% <76.02%> (+2.16%)	⬆️

... and 360 files with indirect coverage changes

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