refactor(server): split bootstrap.rs into the boot/ tree

[hubcio]

bootstrap.rs had grown to 5.1k lines holding every boot concern at
once: config and credential loading, listener start-up, partition
recovery, shard thread handles, the metadata handoff and the
cluster topology. None of it could be read or tested in isolation.

Move each concern into its own boot/ module and keep the shard_main
narrative, with its ordering invariants, whole in boot/mod.rs. Fold
three duplications the split exposed. The five-channel stop
fan-out was copied between the listener-failure and normal
shutdown paths; StopSignals owns it now. The rustls crypto provider
was installed at four sites; bootstrap installs it once before any
shard thread exists. The cluster roster carried the config-declared
ports while the HTTP state patched in the bound one, so the two
disagreed under port-0 binds; shard 0 now publishes the bound ports
into the one roster, and current_config.toml gains the bound HTTP
address so a harness can discover an ephemeral HTTP port like the
other transports.

New integration tests pin the iggy-view header on success, denial,
ping and follower relay, and bound-port discovery under port-0
binds.

[codecov]

Codecov Report

❌ Patch coverage is 81.29353% with 564 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.98%. Comparing base (bbfad59) to head (4bdf28d).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/server/src/boot/credentials.rs	55.59%	128 Missing and 3 partials ⚠️
core/server/src/boot/recovery.rs	83.33%	96 Missing and 13 partials ⚠️
core/server/src/boot/threads.rs	71.62%	93 Missing and 8 partials ⚠️
core/server/src/boot/listeners.rs	77.21%	72 Missing and 18 partials ⚠️
core/server/src/boot/mod.rs	86.85%	56 Missing and 15 partials ⚠️
core/server/src/boot/topology.rs	92.16%	22 Missing and 15 partials ⚠️
core/server/src/boot/handoff.rs	90.05%	14 Missing and 4 partials ⚠️
core/server/src/http.rs	90.90%	3 Missing and 1 partial ⚠️
core/server/src/http/forward.rs	66.66%	2 Missing ⚠️
core/server/src/config_writer.rs	96.15%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4031      +/-   ##
============================================
+ Coverage     84.97%   84.98%   +0.01%     
  Complexity     1402     1402              
============================================
  Files          1230     1236       +6     
  Lines        181408   181465      +57     
  Branches     147703   147761      +58     
============================================
+ Hits         154157   154224      +67     
+ Misses        23199    23164      -35     
- Partials       4052     4077      +25     

Components	Coverage Δ
Rust Core	86.02% <81.29%> (+<0.01%)	⬆️
Java SDK	67.29% <ø> (ø)
C# SDK	75.46% <ø> (ø)
Python SDK	90.06% <ø> (ø)
PHP SDK	85.65% <ø> (ø)
Node SDK	94.50% <ø> (+0.11%)	⬆️
Go SDK	69.35% <ø> (-0.02%)	⬇️

Files with missing lines	Coverage Δ
core/metadata/src/impls/metadata.rs	88.05% <ø> (+0.03%)	⬆️
core/server/src/cluster_meta.rs	99.20% <100.00%> (+0.13%)	⬆️
core/server/src/dispatch/mod.rs	92.27% <100.00%> (+<0.01%)	⬆️
core/server/src/http/error.rs	87.00% <100.00%> (+0.03%)	⬆️
core/server/src/http/state.rs	88.42% <ø> (ø)
core/server/src/http/tls.rs	84.74% <100.00%> (ø)
core/server/src/main.rs	66.66% <ø> (ø)
core/server/src/partition_helpers.rs	76.04% <ø> (ø)
core/server/src/server_error.rs	37.25% <ø> (ø)
core/server/src/shell.rs	90.74% <ø> (ø)
... and 11 more

... and 38 files with indirect coverage changes

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