Add SigmaStudio manual chapter and align EQ runtime with export map.

Document the ADAU1701 schematic chain (sigma-chain.jpg), cross-link from
hardware/firmware chapters, skip safeload on fixed high-pass band 0, and
match EqProfile default peaking frequencies to SigmaStudio.

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

Software/components/drivers/adau1701/include/adau1701/Adau1701Error.hpp

@@ -30,6 +30,7 @@ enum class Adau1701Error {
     DownloadFailed,
     NotBooted,
     SafeloadFailed,
+    InvalidParameter,
 };
 
 } // namespace adau1701

Software/components/drivers/adau1701/src/Adau1701Driver.cpp

@@ -38,6 +38,8 @@ namespace adau1701 {
 namespace {
 constexpr char kTag[] = "Adau1701";
 constexpr int kI2cPort = 0;
+/** Index 0 is the fixed high-pass band (SigmaStudio band 1); not safeloaded. */
+constexpr std::uint8_t kFixedHighPassBandIndex = 0U;
 } // namespace
 
 Adau1701Driver::Adau1701Driver(Adau1701Pins pins)
@@ -197,6 +199,10 @@ std::expected<void, Adau1701Error> Adau1701Driver::applyMixer(
 std::expected<void, Adau1701Error> Adau1701Driver::setEqBand(
     core::EqBandIndex band, core::GainDb gain, core::FrequencyHz center, float q)
 {
+    if (band.value() == kFixedHighPassBandIndex) {
+        return std::unexpected(Adau1701Error::InvalidParameter);
+    }
+
     if (auto ready = ensureBooted(); !ready) {
         return ready;
     }
@@ -227,6 +233,9 @@ std::expected<void, Adau1701Error> Adau1701Driver::applyEq(
     }
 
     for (std::uint8_t i = 0; i < core::EqBandIndex::kBandCount; ++i) {
+        if (i == kFixedHighPassBandIndex) {
+            continue;
+        }
         const auto index = core::EqBandIndex::tryFromIndex(i);
         if (!index) {
             return std::unexpected(Adau1701Error::SafeloadFailed);

Software/components/drivers/adau1701/src/Adau1701Dsp.cpp

@@ -25,6 +25,8 @@ core::DspError Adau1701Dsp::mapError(Adau1701Error error) noexcept
     switch (error) {
     case Adau1701Error::NotBooted:
         return core::DspError::NotBooted;
+    case Adau1701Error::InvalidParameter:
+        return core::DspError::InvalidParameter;
     case Adau1701Error::SafeloadFailed:
         return core::DspError::SafeloadFailed;
     default: