Add Legacy-rate bounded Age lightning and explosions (#69)

docs/INSTABILITY_RUNTIME.md

@@ -6,4 +6,14 @@ The saved provider IDs are independent of installed handlers. A missing optional
 
 The server scheduler runs only for loaded `rivencraft:age/*` levels with a player present and an Age definition. It uses only chunks already loaded near those players, never requests a chunk, and rotates among eligible Ages when the per-tick cap is reached. Configuration bounds the cooldown (minimum 20 ticks), active Ages, handlers and actions per Age, effective provider level, and examined loaded chunks. A handler must claim a shared action permit before each effect. A throwing handler is logged and quarantined temporarily without stopping the server. World-mutating handlers are enabled by default to match Legacy; the setting can still disable them. These limits are guards for upcoming providers, not proof that an arbitrary third-party handler cannot misuse world APIs.
 
-Issue #67 registers the non-world-mutating potion handlers; see [status-effect policy](INSTABILITY_STATUS_EFFECTS.md). Issue #68 adds configurable [colored decay](INSTABILITY_DECAY.md), enabled by default for newly generated configs. Other world hazards remain for #69–#70. The runtime rejects Overworld and preview dimension IDs; its player requirement excludes unattended snapshot chunk loaders. GameTests cover deterministic selection, save round-trips, score-only migration, unknown IDs, scope checks, fair admission, and action caps.
+Issue #67 registers the non-world-mutating potion handlers; see [status-effect policy](INSTABILITY_STATUS_EFFECTS.md). Issue #68 adds configurable [colored decay](INSTABILITY_DECAY.md), enabled by default for newly generated configs. Issue #69 adds lightning and explosions; the remaining hazards belong to #70. The runtime rejects Overworld and preview dimension IDs; its player requirement excludes unattended snapshot chunk loaders. GameTests cover deterministic selection, save round-trips, score-only migration, unknown IDs, scope checks, fair admission, and action caps.
+
+# Lightning and explosions (#69)
+
+The `lightning` and `explosions` cards now run only in loaded Rivencraft Ages with players. Legacy rolled once per chunk per tick: explosions at 1/1000 and lightning at 1/5000 during thunder or 1/100000 otherwise. Each selected hazard card gets an independent roll every scheduler pass, rather than waiting for the rotating two-provider sample. The roll uses up to 81 already-loaded chunks near Age players and the exact probability of at least one Legacy opportunity over that many chunk-ticks. It permits at most one event per hazard type per pass. A per-Age saved sequence and last-event time prevent the same roll/event from replaying after a restart. Lightning also skips an existing bolt at the target.
+
+Legacy used a real lightning bolt and a flaming, block-damaging strength-3 explosion. Both remain enabled by default. Unlike Legacy, the bolt is visual-only and its damage/fire are applied through separate bounded paths. The explosion uses NeoForge's normal explosion hooks plus break-event checks, with a calculator that restricts destruction to one already-loaded chunk, protects spawn, block entities, portals, decay-protected blocks and Rivencraft infrastructure, and caps affected blocks and entity damage. The visual/audio explosion still occurs when griefing is disabled. `mobGriefing` and `doFireTick` gate block changes and ignition; survival/adventure player damage is separately configurable. These are deliberate modern server-safety differences from Legacy.
+
+Server config `instability_hazards` controls enablement, block damage, player damage, fire, interval, maximum nearby loaded chunks, maximum destroyed blocks, damaged entities, damage per entity, and explosion radius. `instability.world_mutation=false` also suppresses block damage and fire without suppressing visuals or entity damage. No hazard is dispatched into preview, Overworld, or another Age.
+
+For an operator smoke test inside an Age, `/rivencraft age hazard pulse lightning` and `/rivencraft age hazard pulse explosion` force one event in a safe loaded chunk. They still honor the Age scope, enable switch, cooldown, spawn exclusion, game rules and damage policies. Repeat after `minimum_gap_ticks` (default 20 ticks); run the same command in the Overworld to verify that it refuses to fire. The earlier 200-tick `min_interval_ticks` test key is retired; existing configs receive the new default when NeoForge corrects them.