fix(triggers): registration follows every flag and list change; migrator honours Running=false; no nullable value types

[kamronbatman]

Summary

Closes the trigger-registration follow-ups parked in #1, and removes the last nullable value types from the engine.

• Registration follows every flag and list change. TriggerActivated is now a property whose setter calls EnsureTriggersActive() (serialized slot unchanged, layout byte-identical). TriggerConfigGump adds and removes through the generated helpers (RemoveFromTriggerDefinitionsAt for the clicked row) and re-registers after each edit; SpawnerJsonImporter clears through ClearTriggerDefinitions() so dirty tracking holds. TriggerSystem.ActivateTriggers still has exactly one caller.
• Deactivation no longer gated on the flag. OnStopped and OnDelete call DeactivateTriggers unconditionally (a no-op when nothing is registered), so clearing TriggerActivated after registration can no longer leave a deleted spawner in the trigger system.
• XmlSpawner migrator honours Running="false" on both node forms: the spawner is stopped (it is constructed running) and its triggers registered or not accordingly. Behaviour note: a Running="true" node with zero parsed entries now imports stopped (before, it was left in its constructed running state), which matches D9's "unreproducible encounters import stopped".
• Perf seed and gump fixes found by review. [ModernSpawnerPerfSeed now activates the proximity trigger it adds, so the proximity-dispatch benchmark measures something. TriggerConfigGump writes the time-window definitions with the trigger type names TriggerSystem actually registers (it wrote walltime:/gametime: before, which parsed as unknown while the gump reported success), and clamps the page after deleting the last trigger on a page.
• No nullable value types: XmlSpawnerImporter delays use the TimeSpan.Zero sentinel AddModernEntry already understands; SkillTrigger gets a TimeSpan cooldown overload; ScriptContext.LocationOverride becomes HasLocationOverride + Point3D. The remaining Type? members live only in the legacy YAML/JSON export models (ScriptExportModels, SpawnerExportModels), which the SpawnerDto-only format decision (D5/D6) retires; they are left for that work.
• Docs: CLAUDE.md rule for trigger mutations; dev-docs/architecture.md §2.2; dev-docs/modernuo-prerequisites.md records ModernUO #2636 (SkillEvents.SkillChecked + test-clock IVT) as open; dev-docs/product-spec.md corrects the D3 row: skill: definitions parse and register today but never fire, because nothing calls ModernSpawnerEvents.OnSkillUsed until the wiring PR subscribes to #2636's event.

Test plan

• New world-backed tests in the sequential collection: toggling TriggerActivated registers/unregisters; adding a definition to an activated running spawner registers immediately; deleting an activated spawner leaves nothing registered; stopping with a stale registration unregisters; deleting a stopped spawner with a stale registration unregisters; migrator Running="false" on both node forms. Each mutation-checked against its reverted behaviour.
• dotnet build ModernSpawner.slnx clean; full suite green (438). The test fixture now runs EventScheduler.Configure() like production, since window triggers schedule through it.
• CI

Follow-ups

• TriggerSystem.OnMobileProximity allocates a TriggerContext class per movement event per spawner; the D2 trigger-state-machine work should make it a readonly struct passed by in.
• Legacy export models keep their nullable members until the SpawnerDto-only format (D5/D6) retires them.