feat(message_bus): read replica sockets ahead, bypassing large frames

[hubcio]

The link shard that owns a peer's replica socket burns a full
core under sustained replicated writes. Nothing batches its reads:
read_message issues one io_uring read for the 256-byte header and
a second for the body. Bytes already sitting in the kernel receive
queue wait for the next call.

A read-ahead buffer sized by message_bus.replica_read_buffer_size
lets one socket read serve a whole burst. It adds no latency, because
the reader never waits for the buffer to fill. One fill delivers
at most one buffer, so a body above that size crosses the buffer in
buffer-sized pieces and pays one extra pass over the payload and one
read per piece. A read that large goes straight into the frame's own
buffer instead.

Zero keeps the unbuffered path, so the A/B baseline is a config change
and not a separate build. The plaintext writer already coalesces into
one writev, so it is left alone.

replica_socket_reads_total and replica_inbound_frames_total carry the
evidence on the real workload: their ratio is the batching factor,
and only link shards bump them. A read counts once it completes,
so a link torn down mid-read cannot inflate the ratio.

[codecov]

Codecov Report

❌ Patch coverage is 99.32432% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.83%. Comparing base (c115986) to head (f980b26).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/message_bus/src/framing.rs	97.77%	0 Missing and 1 partial ⚠️
core/message_bus/src/transports/tcp.rs	99.31%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #4224       +/-   ##
=============================================
- Coverage     87.50%   73.83%   -13.67%     
- Complexity     1575     1576        +1     
=============================================
  Files          1280     1278        -2     
  Lines        223241   203774    -19467     
  Branches     186604   167137    -19467     
=============================================
- Hits         195337   150448    -44889     
- Misses        23193    48668    +25475     
+ Partials       4711     4658       -53     

Components	Coverage Δ
Rust Core	71.77% <99.32%> (-16.80%)	⬇️
Java SDK	68.70% <ø> (+0.01%)	⬆️
C# SDK	77.57% <ø> (+0.11%)	⬆️
Python SDK	90.97% <ø> (ø)
PHP SDK	85.67% <ø> (ø)
Node SDK	96.43% <ø> (-0.02%)	⬇️
Go SDK	70.11% <ø> (-0.06%)	⬇️

Files with missing lines	Coverage Δ
core/configs/src/server_config/defaults.rs	100.00% <100.00%> (ø)
core/configs/src/server_config/displays.rs	100.00% <ø> (ø)
core/configs/src/server_config/message_bus.rs	98.85% <100.00%> (+0.47%)	⬆️
core/message_bus/src/config.rs	77.64% <100.00%> (+0.53%)	⬆️
core/message_bus/src/installer/replica.rs	88.56% <100.00%> (+0.20%)	⬆️
core/message_bus/src/lib.rs	97.52% <100.00%> (-0.06%)	⬇️
core/shard/src/lib.rs	85.51% <100.00%> (-0.03%)	⬇️
core/shard/src/metrics.rs	91.62% <100.00%> (+0.20%)	⬆️
core/message_bus/src/framing.rs	92.96% <97.77%> (+2.60%)	⬆️
core/message_bus/src/transports/tcp.rs	97.91% <99.31%> (-0.74%)	⬇️

... and 296 files with indirect coverage changes

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