feat(io): reduce io_uring initialization log noise

[Coldwings]

Description

Reduces repeated startup noise by moving the two io_uring initialization
diagnostics emitted per io_context from INFO to DEBUG.

Warnings and errors remain unchanged, including auto-detection fallback
warnings. Explicit backend selection remains observable at DEBUG level.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (optimization that improves speed/memory usage)
• Documentation (changes to documentation, comments, or examples)
• Refactoring (code changes that neither fix bugs nor add features)
• Tests (adding or modifying tests)
• Build/CI (changes to build system, CI configuration, or dependencies)

Related Issues

Closes #1011

Changes Made

Core Changes

• Demote the io_uring backend initialization diagnostic to DEBUG.
• Demote auto-detected and explicitly selected io_uring context diagnostics to DEBUG.
• Leave fallback warnings and epoll logging behavior unchanged.
• Record the user-visible logging change in the Unreleased changelog.

API Changes (if applicable)

No API change. Default INFO output becomes quieter; the same diagnostics remain
available at DEBUG level.

Testing

Unit Tests

• Added new tests for the changes
• Updated existing tests if needed
• All tests pass locally

Integration Tests

• Tested with existing examples
• Tested in real-world scenarios (if applicable)

Sanitizer Testing

• Tested with ASAN (AddressSanitizer)
• Tested with TSAN (ThreadSanitizer)
• No new warnings or errors

Test Results

git diff --check origin/main...HEAD
# passed

No local build or runtime test was performed. GitHub Actions provides
executable validation. The change is limited to three log-level call sites and
the changelog.

Checklist

Code Quality

• My code follows the project's code style
• I have added/updated comments for complex logic
• I have removed any debug code, TODOs, or commented-out code
• My changes generate no new warnings

Documentation

• I have updated documentation
• I have added examples for new features (if applicable)
• I have updated API documentation (if applicable)

Testing

• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes
• I have tested with ASAN and TSAN

Compatibility

• My changes are backward compatible
• I have considered the impact on existing users
• I have updated CHANGELOG.md

Performance (if applicable)

• I have considered the performance impact
• I have added benchmarks for performance-critical changes

Reviewer Guidance

Areas requiring special attention:

• Confirm that the two noisy io_uring initialization lines are the only behavior changed.
• Confirm that warnings and operational failures remain visible at default verbosity.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Coldwings]

Independent read-only review summary:

• Scope: the current PR head against its main base.
• Lens: logging behavior, compile-time logging macros, compatibility, changelog accuracy, and unintended backend changes.
• Blocking findings: none.
• Non-blocking findings: none.
• Residual validation gap: the review was static and no dedicated log-capture regression test was added. GitHub Actions remains the executable validation gate.

Verdict: no blocking objections remain for the reviewed head.