Extensions: basic combat as a worked example

The combat extension shows what "a plugin built on the standard layers" means: it invents nothing. One call in a TypeScript story's world setup registers an interceptor on the combatant trait for the attacking action (the exact documented seam), plus an NPC-side resolver for retaliation. From there, everything it touches is standard: hit chance from combatant.skill vs the defender's (clamped 10–95%, weapon skillBonus on top), damage through the health trait's behavior, death and inventory-dropping through the standard death machinery, narration through the attacking-combat-* alias family the lang layer already carries. Its dice are seeded but run as their own stream, so combat outcomes vary run to run, by policy.

A story wanting different combat registers its own interceptor on the same seam instead (Dungeo's melee does). And the boundary the audit once flagged is closed (ADR-215): a Chord story reaches all of this with one header line, use combat, which runs the same registration call at load. With it, combatant and weapon join the composable catalog with their typed fields (skill, base-damage, hostile, weapon damage and skill-bonus, …), and health/max-health land on the health trait like everywhere else. The use line resolves against a fixed, trusted, runtime-bundled registry: an unknown name is a load error, and a use-only story stays pure IR. Third-party extensions remain a deferred design.