Logger subsystem refactoring for concurrent multi-mode output

[HeatCrab]

The logger subsystem needs architectural improvement to reliably handle concurrent output from M-mode and U-mode tasks.

These issues were identified during U-mode(#62) and PMP(#32) development:

• Output Interleaving due to Unprotected I/O Loops #64
• DoS vulnerability in sys_tputs through unbounded NOSCHED blocking #65
• Non-monotonic output timeline in mixed M-mode/U-mode execution #66

These issues are not independent bugs—they stem from a fundamental design tension: async logging via ring buffer and direct UART output via sys_tputs coexist without synchronization. Patching any single issue in isolation is likely to require changes that affect the others.

A proper fix will involve rethinking how output paths interact, which means the logger architecture needs refactoring. This issue tracks the overall effort.

Acceptance Criteria

• All three issues (Output Interleaving due to Unprotected I/O Loops #64, DoS vulnerability in sys_tputs through unbounded NOSCHED blocking #65, Non-monotonic output timeline in mixed M-mode/U-mode execution #66) are resolved
• A dedicated test application validates concurrent output behavior
• No output interleaving when multiple tasks print simultaneously
• Output from M-mode and U-mode tasks appears in execution order