Fix agent-cap wipe, Town03 map crash, and short HDMap smoothing

[cursor]

Bug and impact

Three high-severity correctness bugs on dev:

1. Detection / NPC wipe at the agent cap. PerceptionModel.add_agent_vehicle cleared all agents when c11_max_agents (default 12) was reached, then appended the new one. FastBEV rebuilds detections each frame, so a scene with 13+ clusters left only the last obstacle. Spawning a 13th NPC in BasicSim deleted every previously spawned agent. Lattice / velocity planners then planned through traffic that was still present.

2. Bundled Town03 OpenDRIVE cannot load. _connect_lanes looked up predecessor/successor lanes in the driving-only lane_by_uid map and dereferenced None when the link pointed at a sidewalk or missing lane. After skipping those links, can_laneA_access_laneB still crashed on short CARLA junction centerlines ([:, -3] on a 2-point polyline). Selecting data/Town03_Opt.xodr aborted stack build.

3. Short HDMap routes crash in Savitzky–Golay. smoothen_path_savgol is always called from HDMapGlobalPlanner.plan. After 0.5 m near-duplicate pruning, a same-lane start/goal a few meters apart can leave 3, 4, or 6 points. The window clamp produced window_length > n or polyorder >= window_length, raising ValueError and killing global replan.

Root cause

• Cap handler replaced the list instead of evicting the oldest agent.
• Lane-link resolution assumed every OpenDRIVE predecessor/successor id is a driving lane, and heading checks assumed ≥3 centerline samples.
• Smoothing adjusted window length to an odd number without also enforcing window_length <= n and polyorder < window_length.

Fix

• Evict oldest agents (FIFO) until there is room; refuse adds when the cap is ≤0.
• Skip unresolved lane links; clamp centerline column indices in the access check.
• Clamp Savitzky–Golay window/polyorder and skip smoothing when the cleaned path is too short.

Validation

• test_add_agent_evicts_oldest_when_cap_reached — 5 adds with cap 3 keep the last 3.
• Unresolved-link fixture + bundled Town03_Opt.xodr load without crash.
• smoothen_path_savgol parametrized for n ∈ {2..8}.
• pytest test/c10_perception/ test/c20_planning/test_c24_global_hdmap_planners.py — 28 related tests passed; broader perception/planning subset 43 passed.

Not in this PR

Existing open inspection PRs (#22–#37) were not re-reported. Toolbar Save Settings still omits planning/control YAML (silent profile data loss); F-key profile cycle still skips reload_stack. Those are noted in the Slack summary.