feat(control): track planning path with spline+slerp and PID

[junlinp]

Summary

• assign per-pose timestamps in planning path using trajectory index and dt
• store incoming planning path first in cmd_vel controller and build time-parameterized interpolators
• add cubic spline position interpolation and SLERP orientation interpolation for timestamp sampling
• add two PID loops (position and yaw) to track /slam/odometry against the planning path
• keep stale-path protection and acceleration limiting on /cmd_vel output

Validation

• python -m py_compile tinynav/core/planning_node.py
• python -m py_compile tinynav/platforms/cmd_vel_control.py

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.15 s (264.6 files/s, 39900.0 lines/s)

Language	files	blank	comment	code
Python	12	605	253	3327
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	14	31	7	231
Dockerfile	1	30	29	189
JSON	2	8	0	58
TOML	1	4	0	57
make	1	18	15	55
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	40	915	348	4769

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.15 s (267.4 files/s, 40382.2 lines/s)

Language	files	blank	comment	code
Python	12	606	254	3333
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	14	31	7	231
Dockerfile	1	30	29	189
JSON	2	8	0	58
TOML	1	4	0	57
make	1	18	15	55
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	40	916	349	4775