Fix LaTeX manual build errors for LuaLaTeX.

Rename conflicting TikZ step style to drstep, add missing ch:firmware label, and use TeX Gyre Heros font fallback instead of Linux Biolinum O.

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

Software/docs/manual/ch-adau1701.tex

@@ -114,17 +114,17 @@ the DSP program and writes it into RAM after each reset.
 \begin{figure}[htbp]
   \centering
   \begin{tikzpicture}[
-      step/.style={draw, rounded corners, minimum width=9.5cm,
+      drstep/.style={draw, rounded corners, minimum width=9.5cm,
         minimum height=0.9cm, align=center, font=\small},
       >={Latex}, node distance=3mm]
-    \node[step, fill=black!6] (rst) {Assert RESET\# (10\,ms), release};
+    \node[drstep, fill=black!6] (rst) {Assert RESET\# (10\,ms), release};
-    \node[step, fill=black!6, below=of rst] (i2c)
+    \node[drstep, fill=black!6, below=of rst] (i2c)
       {Create I\textsuperscript{2}C master @ 100\,kHz, address 0x34};
-    \node[step, fill=black!8, below=of i2c] (dl)
+    \node[drstep, fill=black!8, below=of i2c] (dl)
       {Replay \texttt{DspProgram} from flash or embedded export};
-    \node[step, fill=black!6, below=of dl] (ready)
+    \node[drstep, fill=black!6, below=of dl] (ready)
       {\texttt{Adau1701Driver::isBooted()} = true};
-    \node[step, fill=black!10, below=of ready] (prof)
+    \node[drstep, fill=black!10, below=of ready] (prof)
       {\texttt{AudioService::loadAndApply()} --- safeload user profile};
     \foreach \a/\b in {rst/i2c, i2c/dl, dl/ready, ready/prof} {
       \draw[->] (\a) -- (\b);

Software/docs/manual/ch-api.tex

@@ -190,8 +190,8 @@ seek failure.
 \begin{drcode}[JSON]
 {"direction":"up"}
 \end{drcode}
-Use \texttt{"down"} for downward seek. Omit the body or send \texttt{{}}
+Use \texttt{"down"} for downward seek. Omit the body or send an empty JSON
-for upward seek (backward compatible).
+object (\texttt{\{\}}) for upward seek (backward compatible).
 \end{drnote}
 
 \subsection{\texttt{GET /api/audio/profile}}

Software/docs/manual/ch-bt1035.tex

@@ -90,17 +90,17 @@ Line-In path is ready before Wi-Fi starts.
 \begin{figure}[htbp]
   \centering
   \begin{tikzpicture}[
-      step/.style={draw, rounded corners, minimum width=9.5cm,
+      drstep/.style={draw, rounded corners, minimum width=9.5cm,
         minimum height=0.9cm, align=center, font=\small},
       >={Latex}, node distance=3mm]
-    \node[step, fill=black!6] (sys) {SYS\_CTL high, RESET\# pulse};
+    \node[drstep, fill=black!6] (sys) {SYS\_CTL high, RESET\# pulse};
-    \node[step, fill=black!6, below=of sys] (uart)
+    \node[drstep, fill=black!6, below=of sys] (uart)
       {Install UART2 @ 115200, RTS/CTS};
-    \node[step, fill=black!8, below=of uart] (at)
+    \node[drstep, fill=black!8, below=of uart] (at)
       {Send \texttt{AT} --- expect OK};
-    \node[step, fill=black!10, below=of at] (aux)
+    \node[drstep, fill=black!10, below=of at] (aux)
       {Send \texttt{AT+AUXCFG=1} --- expect OK (Line-In)};
-    \node[step, fill=black!6, below=of aux] (done)
+    \node[drstep, fill=black!6, below=of aux] (done)
       {\texttt{Bt1035Driver::isBooted()} = true};
     \foreach \a/\b in {sys/uart, uart/at, at/aux, aux/done} {
       \draw[->] (\a) -- (\b);

Software/docs/manual/ch-firmware.tex

@@ -12,6 +12,7 @@
 % ============================================================
 
 \chapter{Firmware Architecture}
+\label{ch:firmware}
 \label{sec:firmware-architecture}
 
 The DigiRadio firmware runs on the ESP32-S3 and orchestrates three

Software/docs/manual/ch-intro.tex

@@ -4,7 +4,7 @@
 DigiRadio is an open-source, high-fidelity digital radio receiver. It
 receives DAB+ and FM broadcasts, processes the audio through a dedicated
 signal processor, and streams the result over Bluetooth using a
-high-resolution codec. Firmware~0.8.3 on \texttt{main} provides encrypted
+high-resolution codec. Firmware~0.8.4 on \texttt{main} provides encrypted
 storage, a tabbed configuration web UI, and the full REST API documented
 in Chapter~\ref{ch:api}. The whole project --- hardware and firmware --- is
 released as open source for the maker and audio community to study,

Software/docs/manual/ch-si4684.tex

@@ -114,21 +114,21 @@ the \texttt{FLASH\_LOAD} path used by boards with a pre-programmed SST25V.
 \begin{figure}[htbp]
   \centering
   \begin{tikzpicture}[
-      step/.style={draw, rounded corners, minimum width=9cm,
+      drstep/.style={draw, rounded corners, minimum width=9cm,
         minimum height=0.9cm, align=center, font=\small},
       >={Latex}, node distance=3mm]
-    \node[step, fill=black!6] (rst) {Hardware reset (RST\#)};
+    \node[drstep, fill=black!6] (rst) {Hardware reset (RST\#)};
-    \node[step, fill=black!6, below=of rst] (pu)
+    \node[drstep, fill=black!6, below=of rst] (pu)
       {POWER\_UP (crystal / clock args)};
-    \node[step, fill=black!6, below=of pu] (li1) {LOAD\_INIT};
+    \node[drstep, fill=black!6, below=of pu] (li1) {LOAD\_INIT};
-    \node[step, fill=black!8, below=of li1] (patch)
+    \node[drstep, fill=black!8, below=of li1] (patch)
       {HOST\_LOAD \texttt{rom\_patch\_016.bin} (124 B chunks)};
-    \node[step, fill=black!6, below=of patch] (li2) {LOAD\_INIT};
+    \node[drstep, fill=black!6, below=of patch] (li2) {LOAD\_INIT};
-    \node[step, fill=black!8, below=of li2] (img)
+    \node[drstep, fill=black!8, below=of li2] (img)
       {HOST\_LOAD \texttt{dab\_} or \texttt{fm\_firmware.bin}
        (2044 B chunks)};
-    \node[step, fill=black!6, below=of img] (boot) {BOOT};
+    \node[drstep, fill=black!6, below=of img] (boot) {BOOT};
-    \node[step, fill=black!10, below=of boot] (cfg)
+    \node[drstep, fill=black!10, below=of boot] (cfg)
       {Properties: I\textsuperscript{2}S 44.1 kHz, RDS/DAB FE, volume};
     \foreach \a/\b in {rst/pu, pu/li1, li1/patch, patch/li2, li2/img,
       img/boot, boot/cfg} {

Software/docs/manual/digiradio-manual.sty

@@ -35,11 +35,20 @@
   \renewcommand{\familydefault}{\sfdefault} % Biolinum as body (Optima-like)
 \else
   \RequirePackage{fontspec}
-  % Real Optima on macOS; change the name if yours differs.
+  \defaultfontfeatures{Ligatures=TeX}
+  % Real Optima on macOS; TeX Gyre Heros ships with TeX Live as fallback.
   \IfFontExistsTF{Optima}{%
     \setmainfont{Optima}%
   }{%
-    \setmainfont{Linux Biolinum O}% fallback if Optima not installed
+    \IfFontExistsTF{TeX Gyre Heros}{%
+      \setmainfont{TeX Gyre Heros}%
+    }{%
+      \IfFontExistsTF{TeXGyreHeros}{%
+        \setmainfont{TeXGyreHeros}%
+      }{%
+        \setmainfont{Latin Modern Sans}%
+      }%
+    }%
   }
 \fi