feat(server-ng): add partition reconciliation loop

[krishvishal]

What

A level-triggered partition reconciliation loop, one task per shard, that converges each shard's local partition set to the committed Streams metadata.

• Wakes on a post-commit notifier broadcast by shard 0 for partition-shaping ops, a periodic safety tick (default 1s, capped 30s, configurable), and an initial startup pass.
• Each pass: build missing owned partitions, seed routing rows for peer-owned namespaces, tear down ghosts removed from the committed target.

How

Single-writer via the pump funnel. The reconciler runs off-pump. It stages ReconcileOp::{InsertOwned, InsertRouted, ConfirmRemove, RemoveRouted} and the pump applies them single-writer.

Two-phase tombstone teardown. The reconciler fences writes synchronously (tombstone + shards_table row removal), awaits the disk delete, then enqueues ConfirmRemove. A failed delete stays tombstoned and backed off; the in-memory partition is never dropped while its data still exists.

Epoch / fast-skip model. A monotonic Streams::revision (bumped in the STM on every partition-shaping commit) lets a converged pass skip the O(N) diff when nothing changed. A per-partition created_revision distinguishes a delete +
recreate that reused the same slab-key namespace from the original incarnation, triggering a rebuild instead of serving stale segments.

Failure handling. Per-cause exponential backoff (separate budgets for build vs delete) and recovery for a permanently-wedged tombstone after a failed disk delete.

[codecov]

Codecov Report

❌ Patch coverage is 75.55824% with 405 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.55%. Comparing base (7bf1a24) to head (7bbfc82).

Files with missing lines	Patch %	Lines
core/server-ng/src/partition_helpers.rs	61.03%	109 Missing and 11 partials ⚠️
core/server-ng/src/partition_reconciler.rs	88.39%	80 Missing and 6 partials ⚠️
core/server-ng/src/bootstrap.rs	0.00%	82 Missing ⚠️
core/metadata/src/stm/stream.rs	86.97%	28 Missing ⚠️
core/shard/src/lib.rs	75.78%	21 Missing and 2 partials ⚠️
core/partitions/src/iggy_partitions.rs	69.84%	16 Missing and 3 partials ⚠️
core/shard/src/metrics.rs	42.85%	12 Missing ⚠️
core/shard/src/router.rs	0.00%	12 Missing ⚠️
core/shard/src/shards_table.rs	54.54%	10 Missing ⚠️
core/configs/src/server_config/validators.rs	55.55%	4 Missing ⚠️
... and 5 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3436      +/-   ##
============================================
+ Coverage     74.33%   74.55%   +0.22%     
  Complexity      943      943              
============================================
  Files          1247     1249       +2     
  Lines        122232   123567    +1335     
  Branches      98504    99867    +1363     
============================================
+ Hits          90858    92130    +1272     
- Misses        28413    28415       +2     
- Partials       2961     3022      +61     

Components	Coverage Δ
Rust Core	75.73% <75.55%> (+0.28%)	⬆️
Java SDK	58.44% <ø> (ø)
C# SDK	69.41% <ø> (-0.52%)	⬇️
Python SDK	81.06% <ø> (ø)
PHP SDK	83.57% <ø> (ø)
Node SDK	91.35% <ø> (+0.14%)	⬆️
Go SDK	40.25% <ø> (ø)

Files with missing lines	Coverage Δ
core/configs/src/server_config/sharding.rs	85.44% <100.00%> (+0.37%)	⬆️
core/message_bus/src/lib.rs	89.19% <100.00%> (+0.78%)	⬆️
core/metadata/src/impls/metadata.rs	39.31% <100.00%> (+4.07%)	⬆️
core/metadata/src/stm/snapshot.rs	86.01% <100.00%> (+0.18%)	⬆️
core/server/src/main.rs	63.25% <100.00%> (ø)
core/server/src/shard/system/partitions.rs	78.43% <100.00%> (ø)
.../server_common/src/segment_storage/index_writer.rs	73.97% <100.00%> (+1.11%)	⬆️
...rver_common/src/segment_storage/messages_writer.rs	71.05% <100.00%> (+1.18%)	⬆️
...e/server_common/src/sharding/partition_location.rs	100.00% <100.00%> (ø)
core/simulator/src/replica.rs	100.00% <100.00%> (ø)
... and 15 more

... and 37 files with indirect coverage changes

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[hubcio]

overall all these comments are just nits. gj krishna, despite the size change is very reasonable.

[krishvishal]

/ready