feat(engine): voice pipeline Phase 1 — composition engine and data model

Add the voice pipeline composition engine (D-138 Spike 2, Phase 1):

- voice/prompt_builder.rs: full prompt assembly from culture profile,
  tell state, and base text. Handles occasional injection gating,
  epistemic marker extraction, content-length-gated tell injection.
  16 unit tests.

- blueprint.rs: CultureProfile gains voice_persona, voice_examples,
  occasional_injections fields. NpcBlueprint gains tell_behaviors.
  OccasionalInjection struct with kind discriminator (oath/faith/
  hesitancy/etc), frequency, and tell-suppression gating.

- culture-krenn.ron: v2 voice injector from Spike 1 — persona block,
  3 examples, oath injection at 0.25 frequency.

- D-138 amended: Phi-3 dropped entirely, exact Gemma 2B provenance
  documented. Model file renamed to gemma2.gguf.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/lib.rs

@@ -9,6 +9,7 @@ pub mod npc;
 pub mod perception;
 pub mod simulation;
 pub mod storyteller;
+pub mod voice;
 // test_world::reset is always compiled (used by simulation::input).
 // Room definitions, constants, and setup_gauntlet are gated behind
 // the "gauntlet" feature (default-on) to allow stripping from release builds.