fix(symbol): \arg\max/\arg\min 改用 \argmax/\argmin 算子

preamble 已用 \DeclareMathOperator*{\argmax}{arg\,max} 定义算子，应整体使用
而非 \arg\max（后者下标只落在 max 下、且非整体算子）。全文 12 处统一为
\argmax/\argmin。

parts/C-classicalDoa.tex

@@ -124,7 +124,7 @@ The \gls{doa} estimates are then obtained by locating the $K$ dominant peaks of
 \begin{equation}
 \hat{\theta}_k
 =
-\arg\max_{\theta}
+\argmax_{\theta}
 \widehat{P}_{\mathrm{MVDR}}(\theta),
 \qquad k=1,\dots,K.
 \label{eq:mvdr_estimation}
@@ -256,7 +256,7 @@ The \gls{doa} estimates are then obtained by locating the $K$ dominant peaks of
 \begin{equation}
 \hat{\theta}_k
 =
-\arg\max_{\theta}
+\argmax_{\theta}
 P_{\mathrm{MUSIC}}(\theta),
 \qquad k=1,\dots,K.
 \label{eq:music_estimation}
@@ -427,7 +427,7 @@ In practice, $\bm{\Psi}$ can be estimated by solving the least-squares problem
 \begin{equation}
 \widehat{\bm{\Psi}}
 =
-\arg\min_{\bm{\Psi}}
+\argmin_{\bm{\Psi}}
 \left\|
 \mathbf{U}_{s,2} - \mathbf{U}_{s,1}\bm{\Psi}
 \right\|_{\mathrm{F}}^2,
@@ -619,7 +619,7 @@ The joint estimates of \gls{doa} and Doppler are then obtained by searching for
 \begin{equation}
 (\hat{\theta}_k,\hat{\nu}_k)
 =
-\arg\max_{\theta,\nu}
+\argmax_{\theta,\nu}
 P_{\mathrm{ST\text{-}MUSIC}}(\theta,\nu),
 \qquad k=1,\dots,K.
 \label{eq:st_music_estimation}