Player HP mechanic

EG: player should have 3 health points (or life points).
The player loses 1 hp if:
- is damaged by an enemy
- goes below a reset threshold (see `reset_behaviour.gd`)

When hp value reaches 0:
- #12 should appear

Start by reading reset_behaviour.gd and issue #12, then trace how the player currently receives enemy damage and reset-threshold events. Done means the player has the specified health behavior, loses one point for each listed condition, and shows the behavior described by #12 when health reaches zero.

Written by the indexing model from the issue text.