feat!: age void dragons through life stages until they die of old age

[adamico]

Summary

A grown Void Dragon ages by the cargo it unloads, through four life stages, and dies of old age (ADR 0010).

Stage	Speed	Load	Lays eggs
young	×1.0	×1	no
adult	×0.6	×2	yes
senior	×0.35	×3.5	owed eggs only
old	×0.2	×6	no

 Routing.step(dragon at a stop)
   unload → cargo += motes, towardEgg += motes while adult
-  lay eggs every dragonEggMotes
+  lay eggs while adult (owed ones into senior)
+  stop over ∧ empty ∧ outlived ∧ Well loaded where its death shows
+    lay owed eggs → set rider down → death over the Well → record gone, Hatchery.lose(owner)
 speed = stage × max(¼, health/100)

• The egg price counts the owner's living dragons: a death or /voidworks dragon remove lowers it.
• Jade shows the stage, and the presence is sized ×0.8 / ×1 / ×1.1 / ×1.2 by it.
• /voidworks dragon age <stage> sets the dragon an operator looks at to the start of a stage.
• The stage length and factors are config: lifeStageMotes, lifeStageSpeeds, lifeStageLoads.
• Also carries eb8ac1d, a docs commit from main that was never pushed.

Evidence

• Before: a dragon never ages and never dies; every grown dragon lays eggs.
  After: sh ./gradlew build passes 355 JUnit tests (LifeStagesTest, plus RoutingTest for dying only at a loaded stop and no lost eggs, and HatcheryTest). runGameTestServer passes all 225 game tests, including three life_stage_* ones: dying at a Well, a rider set down on the Well it picked, and a rider still mounted through the last unload.
• Seen in the dev client by the maintainer, with short stages: each stage's size, the seat on the rim, and the death beams.

Merge Danger

Door: one-way

Saves change: dragons gain a cargo count, the egg count moves to toward_egg (old counts dropped, dragons start young), and the Hatchery saves gone. Once players lose dragons to old age, that can't be walked back.

Blast Radius: saves

Breaking for Consumers of the API: Routing and VoidDragonRecord constructors change, VoidDragonRecord.speed() moves to LifeStages.speed(dragon), and DragonReport gains a stage.

Closes #54

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