fix(ml,ci): zero-dim guards on all 10 models, eliminate warnings, unblock CI parallelism

- Add dimension validation in DQN, PPO, Mamba2, TGGN, TLOB, Liquid,
  KAN, xLSTM, Diffusion constructors (fail-fast on zero-dim inputs
  that would cause CUDA_ERROR_INVALID_VALUE at runtime)
- Add num_unknown_features > 0 guard to TFT (temporal input required)
- Fix 12 dead-code/unused warnings in test compilation
- Remove opt-level=3 and codegen-units=1 from target rustflags
  (was forcing O3 + single-thread codegen on dev/test builds)
- Remove hardcoded jobs=16 cap (cargo now auto-detects CPU count)
- Switch linker to clang+lld (2-5x faster linking)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/ml/src/diffusion/denoiser.rs

@@ -160,6 +160,15 @@ impl Denoiser {
         vb: VarBuilder<'_>,
         device: &Device,
     ) -> Result<Self, MLError> {
+        if data_dim == 0 || hidden_dim == 0 {
+            return Err(MLError::ConfigError {
+                reason: format!(
+                    "Diffusion denoiser requires data_dim > 0 and hidden_dim > 0 (got {}x{})",
+                    data_dim, hidden_dim
+                ),
+            });
+        }
+
         let time_embed = TimeEmbedding::new(time_embed_dim, hidden_dim, vb.pp("time_embed"))?;
 
         let input_proj = linear(data_dim, hidden_dim, vb.pp("input_proj"))