refactor(neuron): introduce InferenceEvent + wire projection layer

Step 1 of the OpenAI Responses API rollout. Pure refactor — no new
endpoints, no behaviour change on the wire. Lays the seam for
emitting Responses-shaped streaming events from the same harness
output as chat completions in Step 2.

- New `neuron::wire` module tree:
  - `wire::event::InferenceEvent` — format-agnostic enum
    (Start, TextDelta, ReasoningDelta, Finish) the candle harness
    now emits as its native streaming currency.
  - `wire::event::FinishReason` — typed reason that maps cleanly
    onto OpenAI `finish_reason`, OpenAI Responses `status`, and
    Anthropic `stop_reason` strings.
  - `wire::openai_chat::project_chat_stream` — async task that
    consumes an InferenceEvent receiver and produces a
    ChatCompletionChunk receiver, stamping per-request metadata
    (id, created, model_id) onto every chunk. Output matches the
    pre-refactor wire shape bit-for-bit.

- candle.rs refactored to emit InferenceEvent on its internal
  channel through all three streaming paths (CPU
  run_inference_streaming, CUDA single-GPU stream_inference_via_worker,
  CUDA TP chat_completion_tp_stream). The streaming functions lost
  their id/created/model_id parameters since wire-format metadata
  now lives in the projector.

- emit_delta + emit_delta_blocking simplified to single-purpose
  TextDelta emitters with no wire-format coupling.

- chat_completion_stream wraps the InferenceEvent receiver in
  wire_chat::project_chat_stream before returning so the
  /v1/chat/completions HTTP handler keeps consuming
  ChatCompletionChunks unchanged. External signature preserved.

Also fixes a pre-existing helexa-acp test race (three modules each
declared their own static LOCK for HOME mutation, so cross-module
parallelism flaked tests that read HOME at runtime). Consolidated
onto a single crate-wide path_util::ENV_LOCK.

122 helexa-acp tests + 44 neuron tests pass (5 new wire projection
tests). fmt + clippy --workspace -- -D warnings clean. Ran helexa-acp
suite 3x to confirm the env race is closed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/neuron/src/wire/mod.rs

@@ -0,0 +1,23 @@
+//! Wire-format projection layer.
+//!
+//! The candle harness produces a single, format-agnostic stream of
+//! [`InferenceEvent`]s. Each wire format (OpenAI chat completions,
+//! OpenAI Responses, Anthropic messages, …) lives in its own module
+//! under `wire::` and projects that event stream into the chunks /
+//! events its HTTP clients expect.
+//!
+//! The benefit over translating *between* wire shapes (OpenAI chat
+//! → Anthropic, etc.) is that we never have to reason about a
+//! wire-N → wire-M conversion: every translation is wire-N ↔ the
+//! internal event currency, and the projections are independent. A
+//! new wire format adds a new file under `wire::`; nothing else
+//! needs to know about it.
+//!
+//! Today: [`openai_chat`]. Stage 2 adds `openai_responses`. Stage 3
+//! could add a native Anthropic projection that replaces the
+//! gateway-side translation.
+
+pub mod event;
+pub mod openai_chat;
+
+pub use event::{FinishReason, InferenceEvent};