[Portfolio] BattleGrid: cohort rollout

[notion-portfolio-actuator]

This issue was created through the Notion cohort rollout for BattleGrid.

Why this exists

• Move this project out of Notion review mode and into the governed GitHub delivery lane.
• Keep the current next slice tracked in the repo where execution will happen.

Current focus

Run cargo run, verify the happy path, and capture the first blocker.

Done when

• The GitHub issue exists and is linked in the Notion actuation trail.
• GitHub becomes the active delivery surface for the next project slice.