Add RT-Thread thread view provider

[Pandafeng-HHU]

Summary

Add an RT-Thread provider that discovers kernel threads through the RT-Thread object container and renders them in the existing RTOS Views thread table.

Closes #74.

RT-Thread data shown

• thread name, address, and state
• current and initial priority
• remaining and initial time slice
• thread error value
• current stack usage
• peak stack usage from RT-Thread's '#' (0x23) stack fill pattern

Compatibility

• resolves structure offsets from ELF debug information instead of hard-coding them
• supports current RT-Thread sched_thread_ctx fields
• includes fallbacks for legacy direct scheduler fields and current-thread symbols
• uses standard DAP/GDB evaluate and readMemory requests, so it is not tied to a specific debug probe
• protects object-list traversal with cycle and thread-count limits

Validation

• npm run compile
• npm run lint
• VS Code extension-host test: 2 passing
• validated ELF layouts for STM32H743XI and GD32H759IMK6 builds
• validated live object-list traversal on a GD32H759IMK6 target, detecting tshell, tidle0, and timer