Bugs fixed:
1. BUILD_FLAT/BUILD_PROTECTED: the interrupt-context self backtrace
printed the same frames twice.
2. BUILD_KERNEL: a cross-tcb backtrace never included the target
task's kernel stack, so frames on it were missing.
Also reworked the implementation around a list of independent
starting points, each carrying every stack range its fp chain may
cross into, splicing them into one backtrace.
Assisted-by: Claude Code:claude-sonnet-5
Signed-off-by: liang.huang <liang.huang@houmo.ai>
57c7e47a6b