build: add opt-in IWYU support and clean up includes

[fanyang89]

• add opt-in include-what-you-use support via CMake and Taskfile for local include hygiene checks
• exclude generated Cap'n Proto code from IWYU and document the workflow in the README
• fix include ownership issues across core, raftor, tests, and examples so both task iwyu and task test pass

[gemini-code-assist]

Code Review

This pull request implements project-wide "Include What You Use" (IWYU) integration, including CMake support, Taskfile commands, and extensive header cleanup across the library, examples, and tests. It also introduces a mechanism to enable strict compiler warnings and refactors internal logic, such as improving index tracking in the read-only manager using std::optional. Feedback focuses on the migration of the clang-format Docker image to a personal repository, which poses a maintenance risk, and an unnecessary change to a loop counter in the inflights test that introduces signed/unsigned comparison issues.

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.33%. Comparing base (8012291) to head (30bcaf3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
include/raftpp/core/capnp_util.h	93.10%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #63   +/-   ##
=======================================
  Coverage   62.33%   62.33%           
=======================================
  Files          58       58           
  Lines        8491     8489    -2     
  Branches     1425     1425           
=======================================
- Hits         5293     5292    -1     
+ Misses       2588     2586    -2     
- Partials      610      611    +1     

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