feat(nav): execute arrival actions when robot reaches a POI

[xiaolefang-dm]

What

Adds an optional actions field to POI definitions. When the robot arrives at a POI, it executes the action sequence
before advancing to the next waypoint. POIs without actions are completely unaffected.

Why

Navigation to a POI is just half the story — the robot often needs to do something meaningful when it gets there (face
a direction, take a photo, wait). Without this, every use case requires custom code. This gives a clean, declarative
way to attach behaviors to POIs without touching the nav stack.

How it works

pois.json → map_node.pois_data → arrive at POI → action_executor → advance

1. map_node detects POI arrival (existing proximity threshold, unchanged)
2. If the POI has actions, spawns a daemon thread running execute_poi_actions()
3. While actions run, try_publish_nav_path holds — robot stays put
4. After all actions complete, poi_index advances as normal

New file: tinynav/core/action_executor.py

Self-contained, no new dependencies.

Adding a new action type = one async def + one line in ACTION_REGISTRY. No changes to map_node or POI schema needed.

Example pois.json

{
     "1": {
         "id": 1,
         "name": "reception",
         "position": [3.2, 1.5, 0.0],                                                                                      
         "actions": [
           {"type": "lookat", "params": {"target": [5.0, 2.0, 0.0]}},                                                      
           {"type": "photo",  "params": {}},                                                                               
           {"type": "wait",   "params": {"seconds": 3}}
         ]                                                                                                                 
     }                                                       
 }

Backward compatibility

Fully backward compatible. Existing pois.json without actions work as before — no migration needed.

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.17 s (252.5 files/s, 37183.8 lines/s)

Language	files	blank	comment	code
Python	13	613	301	3362
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	30	196
JSON	2	8	0	58
TOML	1	4	0	57
make	1	18	15	55
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	42	934	399	4852

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.17 s (247.0 files/s, 36361.9 lines/s)

Language	files	blank	comment	code
Python	13	613	301	3361
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	30	196
JSON	2	8	0	58
TOML	1	4	0	57
make	1	18	15	55
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	42	934	399	4851