Heart pickup and game over sync, again.

docs/capabilities.md

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+# Capabilities
+
+Capabilities allow several systems provide same functionality in different ways. Exactly one capability implementation should be active.
+
+Capabilities are registered by having an entry in `ctx.cap` table.
+
+# List of capabilities
+
+## `health` capability
+
+Functions:
+ - `health() -> num`
+ - `max_health() -> num`
+ - `set_health(hp: num)`
+ - `set_max_health(hp: num)`
+ - `inflict_damage(dmg: num)`
+
+Provided by:
+ - damage (shared health) system
+
+Used by:
+ - heart pickups (core)