agent: code-atomiser-fix — span propagation across live + model-load spawns (Obs-3)

Before this commit `grep -rIn '#\[instrument\|.instrument(\|in_current_span()'`
returned zero matches across the entire workspace. Every tokio::spawn
and thread::spawn lost its parent span, so structured fields recorded
at the call site (session_id, chunk_id, model_id) did not propagate to
log lines emitted inside the spawn. During concurrent-session incidents
the operator could not correlate a runaway log line back to the request
that started it.

Targeted four highest-value join points:

  * src-tauri/src/commands/live.rs::run_live_session
    #[tracing::instrument(skip_all, fields(session_id, engine, language))]
    Attaches the span to the spawn_blocking worker so every per-chunk
    warning carries the session id that owns it.

  * src-tauri/src/commands/live.rs::maybe_dispatch_chunk
    Manual span attach pattern (#[instrument] can't decorate a closure):
    capture the parent span before thread::spawn, .enter() it on the new
    OS thread, then open an "inference" child span with chunk_id +
    duration_secs. Without this, whisper backend warnings appear
    unparented and a runaway chunk can't be traced back to its session.

  * src-tauri/src/commands/models.rs::ensure_model_loaded
    #[instrument(skip_all, fields(model_id, engine, concurrent))]
    Multi-second load + sequential-GPU guard logs now carry the model
    in flight as a structured field.

  * crates/llm/src/lib.rs::load_model
    #[instrument(skip_all, fields(model_id, use_gpu))]
    Same rationale for LLM loads. Tags llama-backend init lines and
    GPU sequential-guard events with the model identifier.

Storage/audio/hotkey/MCP crates left uninstrumented in this commit —
future sweep. The four sites above are the canonical concurrent-load
correlation points; everything else fans out from them.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/llm/src/lib.rs

@@ -120,6 +120,12 @@ impl LlmEngine {
         self.load_model(LlmModelId::default_tier(), model_path, true)
     }
 
+    // instrument: the load is multi-second (`LlamaBackend::init` +
+    // mmap + GPU layer init). Tagging events with `model_id` and
+    // `use_gpu` lets the operator separate the GPU sequential-guard
+    // logs and llama-backend init lines from the LLM transcription
+    // pipeline by structured field rather than by adjacency.
+    #[tracing::instrument(skip_all, fields(model_id = %model_id.as_str(), use_gpu = use_gpu))]
     pub fn load_model(
         &self,
         model_id: LlmModelId,