feat(transcription): whisper threads use Workload::Whisper + GPU detection

Wires the whisper backend through the new tuning helper. gpu_offloaded
combines a compile-time feature check (whisper-vulkan, in default
features) with a runtime libvulkan probe via the hardware module. If
libvulkan1 is missing on Linux, whisper-rs's vulkan backend silently
falls back to CPU, so we should not reduce threads in that case.

The old magnotia_core::constants::inference_thread_count import is
replaced by tuning::{inference_thread_count, Workload}. The constants
module's helper is removed in Task 6.1 once the LLM call site has
also migrated.

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

Cargo.lock

@@ -2793,7 +2793,6 @@ dependencies = [
  "base64 0.22.1",
  "gdk",
  "gtk",
- "libloading 0.8.9",
  "magnotia-ai-formatting",
  "magnotia-audio",
  "magnotia-cloud-providers",
@@ -2855,12 +2854,14 @@ name = "magnotia-core"
 version = "0.1.0"
 dependencies = [
  "async-trait",
+ "libloading 0.8.9",
  "num_cpus",
  "serde",
  "serde_json",
  "sysinfo",
  "tempfile",
  "thiserror 2.0.18",
+ "tracing",
 ]
 
 [[package]]

crates/transcription/src/whisper_rs_backend.rs

@@ -10,8 +10,9 @@ use std::path::Path;
 
 use whisper_rs::{FullParams, SamplingStrategy, WhisperContext, WhisperContextParameters};
 
-use magnotia_core::constants::inference_thread_count;
 use magnotia_core::error::{MagnotiaError, Result};
+use magnotia_core::hardware::vulkan_loader_available;
+use magnotia_core::tuning::{inference_thread_count, Workload};
 use magnotia_core::types::{Segment, TranscriptionOptions};
 
 use crate::transcriber::{Transcriber, TranscriberCapabilities};
@@ -78,7 +79,8 @@ impl Transcriber for WhisperRsBackend {
                 params.set_initial_prompt(prompt);
             }
         }
-        params.set_n_threads(inference_thread_count() as i32);
+        let gpu_offloaded = cfg!(feature = "whisper-vulkan") && vulkan_loader_available();
+        params.set_n_threads(inference_thread_count(Workload::Whisper, gpu_offloaded) as i32);
         params.set_print_special(false);
         params.set_print_progress(false);
         params.set_print_realtime(false);