Refine solver logging and performance log routing for CBC/XA

[dwr-bli]

Summary
This PR narrows the scope to solver logging and performance observability only.

It updates the CBC/XA logging flow and aligns performance log routing across:

• CbcSolver.java
• XASolver.java
• PerformanceTimer.java
• log4j2.xml

Main updates

• Refine CBC solver logging to use more structured/fluent logger calls.
• Add XA solver lifecycle/performance summary logging for better run visibility.
• Update PerformanceTimer so performance events are routed by solver namespace.
• Update log4j2.xml to support separate CBC/XA detailed logs and separate perf logs.

Scope / Intent
This PR is intentionally limited to logging, observability, and log routing.
It is not intended to change:

• model construction
• solve sequence / fallback order
• tolerance behavior
• warm-start behavior
• numerical results
• solver business logic

Feedback addressed
This version was reduced and adjusted based on earlier review comments, with the goal of keeping the PR focused and low-risk:

• Scope kept to logging/observability only; no tolerant-mode or solver behavior work is included.
• Logging levels were tightened so detailed/internal messages are not broadly exposed as new INFO console output unless they are useful operational lifecycle messages.
• Performance summary logging was simplified toward single structured log entries instead of many separate logging calls.
• MDC-based context handling was not expanded further, to avoid large method reshaping and unnecessary structural churn.
• No extra refactoring was added beyond what was needed for logging/perf routing.
• Diff was kept as narrow as possible around the four files above.
• Message wording was kept more operational and less tied to internal terminology where possible.
• The update preserves existing solver behavior and focuses on observability only.

Design notes

• PerformanceTimer now matches the current log4j2 solver-specific perf routing design.
• log4j2.xml now provides separate detailed/perf log destinations for CBC and XA.
• XA observability additions are intended to make run timing and summary diagnostics easier without altering the XA solve flow.
• CBC logging cleanup is intended to reduce noise and improve consistency, not to redesign the solver.

[dwr-zroy]

Changes requested:

• Additional changes to the use of INFO. See comments.
• Use of private class in XASolver discouraged, it needs to be unit-tested at least.
• Request for class name change (feel free to ignore if you disagree)
• Misc.

[sonarqubecloud]

Quality Gate failed

Failed conditions
6.8% Coverage on New Code (required ≥ 80%)
4.5% Duplication on New Code (required ≤ 3%)
C Maintainability Rating on New Code (required ≥ A)
C Reliability Rating on New Code (required ≥ A)

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[dwr-zroy]

Tested with local run, and performance was good on my machine. Processes didn't linger after the run was over for too long.

Looks good to me.