fix(production): wire real inference path through engine, serving, and streaming

- rtx-inference: sample_next_token now copies the actual logits from the
  forward pass (Tensor::to_vec, last-token slice) instead of sampling
  from a fabricated all-zero vector; request metrics report measured
  queue/processing times instead of hardcoded constants.
- rtx-serving-api: depends on rtx-inference; /v1/completions dispatches
  to a shared InferenceEngine (byte-level tokenization until a real
  tokenizer is threaded through) and returns 503 when no engine is
  loaded instead of mock text. ServingServer::with_engine attaches one.
- rtx-streaming: depends on rtx-inference; generate_tokens delegates to
  an attached backend engine and errors without one instead of emitting
  "token_N" placeholders; tokenization is byte-level, not position-mod.
- speculative decoding: document the orchestration (speculative/) vs
  implementation (medusa.rs/lookahead.rs) layering; CLAUDE.md no longer
  claims a standalone rtx-speculative-decoding crate.

Co-Authored-By: Claude Fable 5 <[email protected]>

crates/production/rtx-inference/src/speculative/mod.rs

@@ -9,6 +9,19 @@
 //! - Configurable acceptance thresholds and draft lengths
 //! - Performance tracking and adaptive optimization
 //! - Safe error handling throughout the pipeline
+//!
+//! # Layering
+//!
+//! This module is the *orchestration* layer: strategy traits
+//! ([`MedusaDraftModel`], [`SelfSpeculativeModel`]), configs, candidate
+//! trees, and the [`AdvancedSpeculativeDecoder`] that combines them. The
+//! concrete numeric implementations live at the crate root:
+//! [`crate::medusa`] (real multi-head FFN prediction over hidden states,
+//! arXiv:2401.10774) and [`crate::lookahead`] (n-gram lookahead decoding).
+//! The configs here (`MedusaConfig`, `LookaheadConfig`) parameterize the
+//! orchestration strategies; they are distinct from the implementation
+//! configs (`crate::medusa::MedusaConfig` is re-exported from the crate
+//! root as `MedusaHeadsConfig` to disambiguate).
 
 mod advanced;
 mod config;