feat(dc_bridge): add unmanaged-shipper mode and atomic config write

[Minipada]

Summary

• Adds shipper.managed (bool, default true): the Bridge's existing spawn/supervise
  behavior stays the default and is unchanged. shipper.managed: false (the split-deployment
  topology, Split deployment: Shipper and Uploader as their own containers, for fleet deployment #440) has the Bridge render the config and connect exactly as before, but locate
  no binary, spawn no child, and install no parent-death signal — an orchestrator owns the
  Shipper's lifecycle instead.
• The rendered config is now written atomically (write to <path>.tmp, then rename()) via a
  new dc_bridge_core::write_file_atomically helper, so a Shipper reading the file — its own
  process in managed mode, an orchestrator-supervised one in unmanaged mode — can never observe
  a partial write.
• shipper.config_path (optional, defaults to the historic temp-file path) makes the config
  location configurable, so it can be placed on a volume shared with the Shipper
  container/pod.
• The ~/ready service already tracked the Shipper via a TCP probe independent of the
  supervisor, so readiness reports correctly in both modes with no logic change there.

Acceptance criteria (issue #444)

• A parameter selects managed (default) or unmanaged shipper supervision
• In unmanaged mode the Bridge locates no binary, spawns no child, and installs no parent-death signal
• In unmanaged mode the Bridge still renders the config and still connects
• The readiness service reports ready only once the Shipper is accepting connections, in both modes
• The rendered config is written atomically (write then rename)
• The config path is configurable, so it can be placed on a shared location
• Managed mode is byte-for-byte unchanged in behaviour; no existing deployment is affected
• Demoable: start a Shipper by hand, point the Bridge at it in unmanaged mode, and Records arrive

Test plan

• prek run --all-files --skip build-doc passes (clang-format, ruff, REUSE, etc.)
• Added AtomicWrite gtest cases to dc_bridge/test/misc_test.cpp (write+rename, overwrite, failure on missing directory)
• CI (colcon test) — this environment has no ROS/colcon workspace to build/test locally; relying on CI
• Manual demo: start vector by hand pointed at shipper.config_path, launch the Bridge with shipper.managed: false, confirm Records arrive

Closes #444

🤖 Generated with Claude Code

https://claude.ai/code/session_01VuumU1P7drY5cr54p3dw9D

[codecov]

Codecov Report

❌ Patch coverage is 19.44444% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.02%. Comparing base (cc95aab) to head (a81c0b8).
⚠️ Report is 5 commits behind head on jazzy.

Files with missing lines	Patch %	Lines
dc_bridge/src/bridge_node.cpp	0.00%	28 Missing ⚠️
dc_bridge/src/atomic_write.cpp	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            jazzy     #454      +/-   ##
==========================================
+ Coverage   70.93%   71.02%   +0.09%     
==========================================
  Files         119      120       +1     
  Lines        7402     7462      +60     
==========================================
+ Hits         5250     5299      +49     
- Misses       2152     2163      +11     

Flag	Coverage Δ
cpp-jazzy	71.02% <19.45%> (+0.09%)	⬆️

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