perf: route whisper + llm n_threads through physical-core helper

Both inference call sites previously called `num_cpus::get()` (logical
thread count). Established whisper.cpp / llama.cpp guidance is that
SMT siblings contend for shared FPU resources during heavy F16/F32
matmul, so going past physical core count anti-scales. Empirical
sweep on Whisper Tiny / 11s JFK clip / Ryzen 5 4650U (6c12t):

  n_threads | xc_time | RTF    | speedup_vs_1
  ----------|---------|--------|-------------
          1 |   0.33s |  0.030 |   1.00x
          2 |   0.33s |  0.030 |   1.00x
          4 |   0.30s |  0.028 |   1.09x
          6 |   0.32s |  0.029 |   1.04x
          8 |   0.31s |  0.028 |   1.06x
         12 |   0.32s |  0.029 |   1.03x

Tiny doesn't scale (work dominated by overhead) but the larger
Whisper variants and Qwen LLMs do. Sources: whisper.cpp #200, #1033,
#1252, #403; llama.cpp #3167, #572.

Changes:
- crates/core/src/constants.rs:
  * MIN_INFERENCE_THREADS lowered 4 → 2 (research-derived floor)
  * MAX_INFERENCE_THREADS = 8 added (research-derived ceiling)
  * inference_thread_count() rewritten:
      - reads MAGNOTIA_INFERENCE_THREADS env var (override)
      - num_cpus::get_physical() with available_parallelism fallback
      - clamped to [MIN_INFERENCE_THREADS, MAX_INFERENCE_THREADS]
- crates/core/Cargo.toml: + num_cpus = "1"
- crates/transcription/src/whisper_rs_backend.rs: call site uses helper.
- crates/llm/src/lib.rs: call site uses helper.
- crates/transcription/Cargo.toml: drop num_cpus from [features] +
  [dependencies] (production no longer needs it). Move to
  [dev-dependencies] for tests/thread_sweep.rs only.
- crates/llm/Cargo.toml: drop num_cpus = "1" (no longer used directly).

Per-machine maps (after this patch):
  Ryzen 5 4650U  6c12t → 6
  big-iron 12c24t       → 8 (clamp; users can override)
  cheap 2c2t laptop     → 2
  1c container/VM       → 2

Adds crates/transcription/tests/thread_sweep.rs — env-gated like
jfk_bench, prints the table above against any model + WAV. Useful for
re-baselining on new hardware or when tuning the clamp values.

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

crates/transcription/tests/thread_sweep.rs

@@ -0,0 +1,76 @@
+//! Thread-count scaling sweep for Whisper Tiny.
+//! Runs the JFK clip at n_threads = 1, 2, 4, 6, 8, 12, prints RTF table.
+//! Gated on the same env vars as jfk_bench.
+
+use std::env;
+use std::time::Instant;
+
+#[test]
+fn whisper_thread_count_sweep() {
+    let Ok(model_path) = env::var("MAGNOTIA_WHISPER_TEST_MODEL") else { return };
+    let Ok(audio_path) = env::var("MAGNOTIA_WHISPER_TEST_AUDIO") else { return };
+
+    use whisper_rs::{FullParams, SamplingStrategy, WhisperContext, WhisperContextParameters};
+
+    let bytes = std::fs::read(&audio_path).expect("read wav");
+    let sample_rate = u32::from_le_bytes(bytes[24..28].try_into().unwrap());
+    let pcm = &bytes[44..];
+    let samples: Vec<f32> = pcm
+        .chunks_exact(2)
+        .map(|c| i16::from_le_bytes([c[0], c[1]]) as f32 / 32768.0)
+        .collect();
+    let audio_secs = samples.len() as f64 / sample_rate as f64;
+    eprintln!("[sweep] audio: {:.2}s @ {} Hz", audio_secs, sample_rate);
+
+    let logical = num_cpus::get();
+    let physical = num_cpus::get_physical();
+    eprintln!("[sweep] CPU: physical={}, logical={}", physical, logical);
+
+    let ctx = WhisperContext::new_with_params(&model_path, WhisperContextParameters::default())
+        .expect("model load");
+
+    // Warm-up pass at default to prime caches
+    {
+        let mut state = ctx.create_state().expect("state");
+        let mut params = FullParams::new(SamplingStrategy::Greedy { best_of: 1 });
+        params.set_language(Some("en"));
+        params.set_n_threads(physical as i32);
+        params.set_print_special(false);
+        params.set_print_progress(false);
+        params.set_print_realtime(false);
+        state.full(params, &samples).expect("warmup");
+    }
+
+    let mut targets: Vec<i32> = vec![1, 2, 4, physical as i32, logical as i32];
+    if logical >= 8 && !targets.contains(&8) { targets.push(8); }
+    targets.sort();
+    targets.dedup();
+
+    eprintln!("");
+    eprintln!("=== n_threads scaling ===");
+    eprintln!("n_threads | xc_time | RTF    | speedup_vs_1");
+    eprintln!("----------|---------|--------|-------------");
+    let mut baseline_dur: Option<f64> = None;
+    for n in &targets {
+        // Two runs, take the min (deeper of L2/L3 effects; we want best-case)
+        let mut best = f64::MAX;
+        for _ in 0..2 {
+            let mut state = ctx.create_state().expect("state");
+            let mut params = FullParams::new(SamplingStrategy::Greedy { best_of: 1 });
+            params.set_language(Some("en"));
+            params.set_n_threads(*n);
+            params.set_print_special(false);
+            params.set_print_progress(false);
+            params.set_print_realtime(false);
+            let t = Instant::now();
+            state.full(params, &samples).expect("transcribe");
+            let dur = t.elapsed().as_secs_f64();
+            if dur < best { best = dur; }
+        }
+        let rtf = best / audio_secs;
+        let speedup = baseline_dur.map(|b| b / best).unwrap_or(1.0);
+        if baseline_dur.is_none() { baseline_dur = Some(best); }
+        eprintln!("{:>9} | {:>6.2}s | {:>6.3} | {:>6.2}x",
+            n, best, rtf, speedup);
+    }
+}