perf(server-ng): cap sealed-segment read handles with per-partition LRU

[hubcio]

A lone consumer crossing a sealed segment degraded to full-segment
scans: rotation drops the writer and the resident index, so every poll
re-opened the file and scanned from byte 0. Sealed segments now share
a per-partition read-state handle caching the read fd and the sparse
index reloaded from the .index file, capped by an LRU so descriptors
stay bounded.

Shard allocation defaults back to numa:auto now that multi-shard
server-ng is stable; the previous single-shard default collapsed all
work onto one core.

This is half of the single-node latency parity work; the produce-side
single-pass batch checksum change lives in #3746.

[codecov]

Codecov Report

❌ Patch coverage is 94.54297% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.84%. Comparing base (b452d15) to head (d7b9b3d).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/partitions/src/poll_plan.rs	78.48%	14 Missing and 3 partials ⚠️
core/partitions/src/iggy_partition.rs	96.22%	7 Missing and 6 partials ⚠️
core/partitions/src/log.rs	97.36%	4 Missing and 1 partial ⚠️
core/partitions/src/iggy_index_reader.rs	96.61%	0 Missing and 4 partials ⚠️
core/partitions/src/iggy_partitions.rs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3806       +/-   ##
=============================================
- Coverage     76.01%   32.84%   -43.17%     
  Complexity      969      969               
=============================================
  Files          1325     1323        -2     
  Lines        162380   141977    -20403     
  Branches     135260   114935    -20325     
=============================================
- Hits         123427    46637    -76790     
- Misses        35291    94127    +58836     
+ Partials       3662     1213     -2449     

Components	Coverage Δ
Rust Core	21.93% <94.54%> (-54.01%)	⬇️
Java SDK	62.71% <ø> (ø)
C# SDK	71.13% <ø> (-1.14%)	⬇️
Python SDK	89.45% <ø> (ø)
PHP SDK	84.52% <ø> (ø)
Node SDK	96.24% <ø> (+0.08%)	⬆️
Go SDK	43.08% <ø> (ø)

Files with missing lines	Coverage Δ
core/server-ng/src/dispatch.rs	40.00% <ø> (ø)
core/partitions/src/iggy_partitions.rs	70.02% <50.00%> (-0.34%)	⬇️
core/partitions/src/iggy_index_reader.rs	87.19% <96.61%> (+87.19%)	⬆️
core/partitions/src/log.rs	68.55% <97.36%> (+37.00%)	⬆️
core/partitions/src/iggy_partition.rs	64.27% <96.22%> (+8.09%)	⬆️
core/partitions/src/poll_plan.rs	73.07% <78.48%> (+8.62%)	⬆️

... and 733 files with indirect coverage changes

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