Add partner_blindness_prob (blind-agent robustness knob)

[eugenevinitsky]

Summary

• Ports the blind-agent robustness feature from vcha/turbostream. Each agent has a per-episode probability partner_blindness_prob of being "blind" — its partner observations are zeroed for the whole episode, making it an unpredictable hazard for surrounding traffic.
• Blind agents are masked out of the PPO rollout buffer (GIGAFLOW Appendix B.4) so their transitions don't pollute the gradient.
• Default 0.0 (off), so behavior is unchanged unless you opt in.

Changes

• config/drive.ini: new partner_blindness_prob = 0.0 knob under a [Robustness features] block.
• sim/env_fields.h: wires the kwarg through ENV_FIELDS.
• sim/drive.h: adds partner_blindness_prob to the Drive struct; per-episode sampling in c_reset; early-return in write_partner_obs for blind egos; mask out blind agents in c_step.
• sim/datatypes.h: adds is_blind_partner flag to Agent.

Test plan

• ./build.sh --fast builds clean
• ./build.sh builds clean (torch backend)
• With partner_blindness_prob = 0.0: training metrics unchanged vs. base
• With partner_blindness_prob = 0.05: ~5% of agents per episode see zeroed partner obs and contribute mask=0 entries; check masks distribution in a short rollout
• Sanity: collision rate doesn't spike with blind agents off

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds an opt-in “blind partner observations” robustness feature to the drive simulator, allowing a per-episode fraction of agents to have their partner (other-agent) observations zeroed while excluding their transitions from PPO rollouts to avoid gradient contamination.

Changes:

• Introduces partner_blindness_prob as a new [env] configuration knob (default 0.0).
• Wires the new kwarg through ENV_FIELDS into the Drive struct.
• Implements per-episode sampling of Agent.is_blind_partner, zeros partner observations for blind agents, and sets masks[i]=0 for blind agents in c_step.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
config/drive.ini	Adds the new robustness configuration knob and documentation comments.
sim/env_fields.h	Exposes partner_blindness_prob via the centralized env-kwarg field list.
sim/datatypes.h	Adds an episode-level Agent.is_blind_partner flag.
sim/drive.h	Samples blindness per episode, skips writing partner observations for blind agents, and masks blind agents out of PPO rollouts.

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