Confirm on a real client that the breath bar starts when the head goes under #142

Expected (1.12.1). When a player's head goes under water, the breath bar appears and drains over about a minute. It clears the moment the head breaks the surface. At zero breath the player takes drowning damage, which the combat log names, and can die.

Actual (at 87109e3c). The server side is done. Air drains, health falls, and the timer packets go out (breath.rs#L42). A Headless Client dive adapter in the wire-harness checks the timer start, the health drop, the drowning combat-log line and the timer stop. "Submerged" means the water surface is more than UNDERWATER_HEAD_HEIGHT (2.0 yd) above the feet (env.rs#L10). No behavioural reference gives that value, and nobody has checked it against a real client.

How to check. On a local Realm with imported liquid data, dive in Crystal Lake and Stone Cairn Lake on a real 1.12.1 client:

• The bar appears when the head goes under, not the feet, drains smoothly, and clears on surfacing.
• Drowning damage reads correctly in the combat log, and a drowning death shows the ordinary Release Spirit screen.
• A partial dive that surfaces mid-drain, and a zone change while submerged, leave no stuck bar.

If the bar starts early or late, tune UNDERWATER_HEAD_HEIGHT and dive again.

Done when. The results and the final UNDERWATER_HEAD_HEIGHT value are recorded here, with the client build and the commit.