Add stereo and bass enhance APIs via PEQ overlay.

Expose POST /api/audio/stereo-enhance and bass-enhance with 0–100 levels,
persist enhancements in AudioProfile, and document the virtual EQ mapping in
the manual and SigmaStudio chapter.

Co-authored-by: Cursor <cursoragent@cursor.com>

Software/docs/manual/ch-api.tex

@@ -184,9 +184,11 @@ The handler reads \texttt{audio::AudioService::currentProfile()}.
 {"mixer":{"si4684_left_db":0,"si4684_right_db":0,"esp32_left_db":0,
  "esp32_right_db":0,"mix_left_db":0,"mix_right_db":0},
  "master":{"left_db":0,"right_db":0},
- "eq":[{"gain_db":0,"center_hz":40,"q":1.414}, ...]}
+ "eq":[{"gain_db":0,"center_hz":40,"q":1.414}, ...],
+ "enhancements":{"stereo_level":0,"bass_level":0}}
 \end{drcode}
 Six EQ bands are always present (\texttt{eq} array length~6).
+Enhancement levels are 0--100; at 0 the base EQ band settings apply.
 \end{drnote}
 
 HTTP status: \textbf{200 OK}; \textbf{503} when the audio service is
@@ -217,6 +219,32 @@ applies it to the DSP, and persists to NVS. Success response:
 \texttt{\{"status":"saved"\}}. HTTP status: \textbf{200 OK}; \textbf{500}
 on apply/persist failure.
 
+\subsection{\texttt{POST /api/audio/stereo-enhance}}
+\label{sec:api-audio-stereo-enhance}
+
+Adjusts stereo depth via a psychoacoustic PEQ overlay on bands 3--5
+(1\,kHz / 3\,kHz / 8\,kHz). This is \emph{not} true M/S widening---there
+is no dedicated SigmaStudio widener block; see
+Section~\ref{sec:ss-enhancements}.
+
+\begin{drnote}[Request body]
+\begin{drcode}[JSON]
+{"level":50}
+\end{drcode}
+\texttt{level} is an integer 0--100 (0 = off, 100 = maximum).
+\end{drnote}
+
+Success response: \texttt{\{"status":"saved"\}}. HTTP status:
+\textbf{200 OK}; \textbf{400} for invalid JSON or level; \textbf{500}
+when safeload or NVS persistence fails.
+
+\subsection{\texttt{POST /api/audio/bass-enhance}}
+\label{sec:api-audio-bass-enhance}
+
+Adjusts bass emphasis via a PEQ overlay on bands 1--2 (100\,Hz /
+400\,Hz). Request and response schema match
+\texttt{POST /api/audio/stereo-enhance} (Section~\ref{sec:api-audio-stereo-enhance}).
+
 \section{Boot and network state machine}
 \label{sec:api-boot-flow}
 

Software/docs/manual/ch-classes.tex

@@ -201,7 +201,13 @@ Constructed once in \texttt{HardwareBootstrap} alongside the driver.
 Application service for ADAU1701 mixer, EQ, and master volume. Holds a
 reference to \texttt{core::IDsp}, tracks the in-memory \texttt{AudioProfile},
 loads from \texttt{IAudioProfileStore} after boot, and applies changes via
-safeload. Exposed on \texttt{/api/audio/*} and the web UI Audio section.
+safeload. Stereo depth and bass enhance levels are merged into the effective
+EQ via \texttt{core::applyEnhancementsToEq()}. Exposed on
+\texttt{/api/audio/*} and the web UI Audio section.
+
+\section{EnhanceLevel}\label{cls:EnhanceLevel}
+Strong type for 0--100 enhancement intensity (stereo depth and bass boost).
+Validated at the HTTP boundary by \texttt{core::parseEnhanceLevelJson()}.
 
 \section{NvsAudioProfileStore}\label{cls:NvsAudioProfileStore}
 \texttt{IAudioProfileStore} implementation storing serialised

Software/docs/manual/ch-sigmastudio.tex

@@ -213,6 +213,26 @@ SigmaStudio export (Section~\ref{sec:ss-export}).
   \label{tab:ss-runtime}
 \end{table}
 
+\section{Virtual enhancements (stereo depth and bass boost)}
+\label{sec:ss-enhancements}
+
+The SigmaStudio export does not include dedicated stereo widener or bass
+boost blocks. Firmware~0.5.0 maps enhancement levels (0--100) onto the
+existing Param EQ1 bands at runtime:
+
+\begin{itemize}
+  \item \textbf{Bass enhance} --- peaking boost at 100\,Hz (+9\,dB max)
+        and 400\,Hz (+3\,dB max).
+  \item \textbf{Stereo enhance} --- slight 1\,kHz cut, plus 3\,kHz and
+        8\,kHz lift for a wider, more present image. This is a
+        psychoacoustic curve, not mid/side processing.
+\end{itemize}
+
+Enhancement levels are stored in \texttt{AudioProfile::enhancements} and
+applied by \texttt{core::applyEnhancementsToEq()} before safeload. Base EQ
+band settings in NVS are preserved; overlays replace affected bands only
+while the corresponding level is greater than zero.
+
 \begin{drcaution}[Use safeload for live changes]
 All runtime updates to the volume, source levels, and EQ bands must go
 through the ADAU1701 safeload mechanism. Writing parameter cells directly