feat(server): cooperative partition reassignment for consumer groups

[spetz]

Partitions moved instantly on join — the old consumer was
mid-processing, the new one polled the same uncommitted
offset, causing duplicates. Three separate metadata reads
let a left-right swap present inconsistent snapshots.

Cooperative rebalancing: excess partitions marked "pending
revocation" on join, excluded from round-robin. Transfer
completes when committed >= polled offset or timeout.

Atomic resolution: group, member, and partition lookup
under single metadata read guard. last_polled_offsets
moved to PartitionMeta via Arcpapaya::HashMap.

Key mechanics:

• target_member_id guard against slab slot reuse
• Leave-scoped offset clearing (not entire group)
• TOCTOU re-check on shard 0 before completion
• Release/Acquire ordering on cross-shard atomics
• Zero-alloc fast path when no pending revocations
• Two-phase join with separate completion cycles
• Periodic timeout checker with early-return

Integration tests across Tcp/WebSocket/Quic: reshuffle,
duplicates, auto/manual commit, never-polled transfer,
member leave, target leave, concurrent polls, stale
client, timeout, sequential joins, partition add/delete,
even distribution convergence.

[codecov]

Codecov Report

❌ Patch coverage is 83.54232% with 105 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.75%. Comparing base (153cb37) to head (b0d6c33).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/server/src/metadata/reader.rs	64.33%	51 Missing and 5 partials ⚠️
core/server/src/metadata/consumer_group.rs	87.89%	19 Missing and 4 partials ⚠️
core/server/src/configs/defaults.rs	0.00%	15 Missing ⚠️
core/server/src/shard/system/consumer_offsets.rs	89.28%	1 Missing and 5 partials ⚠️
core/server/src/metadata/writer.rs	89.65%	2 Missing and 1 partial ⚠️
core/server/src/metadata/absorb.rs	96.49%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2734      +/-   ##
============================================
+ Coverage     68.58%   68.75%   +0.17%     
  Complexity      585      585              
============================================
  Files           735      736       +1     
  Lines         60370    60926     +556     
  Branches      56783    57339     +556     
============================================
+ Hits          41403    41889     +486     
- Misses        16941    17005      +64     
- Partials       2026     2032       +6     

Flag	Coverage Δ
rust	70.28% <83.54%> (+0.16%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
core/server/src/bootstrap.rs	80.68% <100.00%> (+0.04%)	⬆️
core/server/src/configs/displays.rs	93.04% <100.00%> (+0.25%)	⬆️
core/server/src/configs/server.rs	66.66% <ø> (ø)
core/server/src/metadata/consumer_group_member.rs	100.00% <100.00%> (ø)
core/server/src/shard/handlers.rs	78.18% <100.00%> (+1.13%)	⬆️
core/server/src/shard/mod.rs	82.56% <100.00%> (+0.27%)	⬆️
core/server/src/shard/system/consumer_groups.rs	85.00% <100.00%> (+14.45%)	⬆️
core/server/src/shard/system/messages.rs	89.18% <100.00%> (+0.90%)	⬆️
core/server/src/shard/system/partitions.rs	84.54% <100.00%> (+0.06%)	⬆️
core/server/src/shard/system/utils.rs	79.50% <100.00%> (+0.21%)	⬆️
... and 9 more

... and 9 files with indirect coverage changes

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