sched/sched: add sched_timer.c to improve tick processing and enable hrtimer support for scheduler

[wangchdo]

Summary

This PR includes two commits:

commit 1:
Consolidate nxsched_process_timer into sched_timer.c, and introduce
nxsched_process_tick() to improve readability and maintainability of
the scheduler timer processing logic.

commit 2:
Add hrtimer support to drive the scheduler.

Impact

Improve scheduler timer processing logic and enable hrtimer support for scheduler.

Testing

ostest passed on rv-virt:smp64 when hrtimer is enabled

NuttShell (NSH)
nsh> 
nsh> 
nsh> uname -a
NuttX 0.0.0 a8c1583552-dirty Jan 19 2026 13:12:31 risc-v rv-virt
nsh> ostest

(...)

user_main: hrtimer test

End of test memory usage:
VARIABLE  BEFORE   AFTER
======== ======== ========
arena     1fc0c20  1fc0c20
ordblks         6        6
mxordblk  1fab270  1fab270
uordblks    10898    12080
fordblks  1fb0388  1faeba0

Final memory usage:
VARIABLE  BEFORE   AFTER
======== ======== ========
arena     1fc0c20  1fc0c20
ordblks         1        6
mxordblk  1fb5b30  1fab270
uordblks     b0f0    12080
fordblks  1fb5b30  1faeba0
user_main: Exiting
ostest_main: Exiting with status 0
nsh> 

ostest passed on a2g-tc397-5v-tft:nsh when hrtimer is enabled

NuttShell (NSH)
nsh>
nsh>
nsh>
nsh> uname -a
NuttX 0.0.0 a8c1583552-dirty Jan 19 2026 13:16:08 tricore a2g-tc397-5v-tft
nsh>
nsh> ostest

(...)

user_main: hrtimer test

End of test memory usage:
VARIABLE  BEFORE   AFTER
======== ======== ========
arena       28dfc    28dfc
ordblks         6        6
mxordblk    1f890    1f890
uordblks     5574     5574
fordblks    23888    23888

Final memory usage:
VARIABLE  BEFORE   AFTER
======== ======== ========
arena       28dfc    28dfc
ordblks         1        6
mxordblk    24208    1f890
uordblks     4bf4     5574
fordblks    24208    23888
user_main: Exiting
ostest_main: Exiting with status 0
nsh>