sched/sleep: Add nxsched_nanosleep() API

[wangchdo]

Summary

Introduce the nxsched_nanosleep() API to provide a lightweight nanosecond-level sleep based on nxsched_ticksleep(). This API offers the same functionality as nxsig_nanosleep() but without signal-related overhead, making it suitable for implementing libc sleep() or usleep() when signals are disabled.

Impact

New api added, no impact to existing nuttx functions

Testing

ostest passed on board fvp-armv8r-aarch32

NuttShell (NSH)
nsh> [ 0] Idle_Task: nx_start: CPU0: Beginning Idle Loop
nsh>
nsh> uname -a
NuttX 0.0.0 3e83236063 Nov 23 2025 23:19:00 arm fvp-armv8r-aarch32
nsh> ostest

(...)

user_main: vfork() test
[ 4] ostest: arm_fork: fork context [0x2000f6b8]:
[ 4] ostest: arm_fork:   r4:20003860 r5:00000000 r6:00000000 r7:00000000
[ 4] ostest: arm_fork:   r8:00000000 r9:00000000 r10:00000000
[ 4] ostest: arm_fork:   r11:00000000 sp:2000f6e0 lr:00055f04
[ 4] ostest: nxtask_setup_fork: Child priority=100 start=0x55f04
[ 4] ostest: nxtask_setup_fork: parent=0x2000d358, returning child=0x2000f790
[ 4] ostest: arm_fork: TCBs: Parent=0x2000d358 Child=0x2000f790
[ 4] ostest: arm_fork: Parent: stackutil:152
[ 4] ostest: arm_fork: Old stack top:2000f778 SP:2000f6e0 FP:00000000
[ 4] ostest: arm_fork: New stack top:20013a80 SP:200139e8 FP:00000000
[ 4] ostest: nxtask_start_fork: Starting Child TCB=0x2000f790
[ 4] ostest: nxtask_activate: ostest pid=85,TCB=0x2000f790
[85] ostest: nxtask_exit: ostest pid=85,TCB=0x2000f790
vfork_test: Child 85 ran successfully

Final memory usage:
VARIABLE  BEFORE   AFTER
======== ======== ========
arena     7ff9d3c  7ff9d3c
ordblks         1        5
mxordblk  7ff0880  7febf08
uordblks     94bc     b55c
fordblks  7ff0880  7fee7e0
user_main: Exiting
[ 4] ostest: nxtask_exit: ostest pid=4,TCB=0x2000d358
ostest_main: Exiting with status 0
stdio_test: Standard I/O Check: fprintf to stderr
[ 3] ostest: nxtask_exit: ostest pid=3,TCB=0x2000af38
nsh>