refactor(server): make dispatch routing and failure paths explicit

[hubcio]

The TCP dispatch funnel hid its rules in control flow: seven
ordered if-return probes where the order was the only spec, two
rewrite chains written out inline in two places, twenty send
sites each picking a failure channel by whichever builder they
called, two client-request handlers on shard 0 with separate
queues, and a read gate that let unknown codes fall through to a
catch-all.

Each is now written down once. classify() returns a RequestClass
that handle_client_request matches on, so the order of checks
inside classify is the routing. rewrite.rs holds both rewrite
chains as pure functions. dispatch/failure.rs holds the failure
channel table and the one exit every host-built frame takes. Each
shard builds one client-request handler that shard 0's transports
share, dropping the duplicate queues, the double hook install and
a reference cycle that leaked the shard. The read gate now
decides every code in the command table instead of passing
unlisted ones to the builder's empty-ok. Tests pin the probe
order, each channel's bytes, and every table entry.

Removing the cycle exposed a shutdown bug it had hidden: shard 0
owns the only write handle to the metadata state machine and
could drop it while peers were still reading, panicking their
pumps. Peers now count themselves out once their runtime is gone,
and shard 0 waits for that, bounded by shutdown_join_timeout.

[codecov]

Codecov Report

❌ Patch coverage is 95.69231% with 70 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.21%. Comparing base (e6ec869) to head (f5da637).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/server/src/dispatch/session_ops.rs	70.96%	18 Missing ⚠️
core/server/src/dispatch/partition.rs	80.00%	16 Missing and 1 partial ⚠️
core/server/src/rewrite.rs	95.17%	12 Missing and 2 partials ⚠️
core/server/src/dispatch/mod.rs	97.40%	11 Missing and 1 partial ⚠️
core/server/src/boot/threads.rs	98.12%	2 Missing and 1 partial ⚠️
core/server/src/boot/mod.rs	90.90%	2 Missing ⚠️
core/server/src/dispatch/failure.rs	99.50%	1 Missing and 1 partial ⚠️
core/server/src/dispatch/authz.rs	98.94%	1 Missing ⚠️
core/server/src/http/submit.rs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4036      +/-   ##
============================================
+ Coverage     85.12%   85.21%   +0.09%     
  Complexity     1402     1402              
============================================
  Files          1236     1238       +2     
  Lines        181607   182489     +882     
  Branches     147900   148782     +882     
============================================
+ Hits         154596   155514     +918     
+ Misses        22957    22911      -46     
- Partials       4054     4064      +10     

Components	Coverage Δ
Rust Core	86.15% <95.69%> (+0.10%)	⬆️
Java SDK	67.29% <ø> (ø)
C# SDK	75.46% <ø> (+0.07%)	⬆️
Python SDK	90.06% <ø> (ø)
PHP SDK	85.65% <ø> (ø)
Node SDK	96.15% <ø> (-0.09%)	⬇️
Go SDK	69.35% <ø> (+0.06%)	⬆️

Files with missing lines	Coverage Δ
core/server/src/boot/recovery.rs	96.01% <100.00%> (+0.05%)	⬆️
core/server/src/dispatch/reads.rs	90.06% <100.00%> (+0.87%)	⬆️
core/server/src/dispatch/test_support.rs	83.33% <100.00%> (+0.87%)	⬆️
core/server/src/http/handlers.rs	79.53% <ø> (ø)
core/server/src/dispatch/authz.rs	98.40% <98.94%> (+3.63%)	⬆️
core/server/src/http/submit.rs	87.09% <50.00%> (-0.85%)	⬇️
core/server/src/boot/mod.rs	86.91% <90.90%> (+0.01%)	⬆️
core/server/src/dispatch/failure.rs	99.50% <99.50%> (ø)
core/server/src/boot/threads.rs	82.52% <98.12%> (+5.97%)	⬆️
core/server/src/dispatch/mod.rs	96.51% <97.40%> (+4.24%)	⬆️
... and 3 more

... and 47 files with indirect coverage changes

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