Add BT1035 driver with manual chapter and expand Si4684 tuning docs.

Implement Bt1035Driver AT init (AT+AUXCFG=1), core AT parser with host tests,
wire boot into HardwareBootstrap, and document DAB/FM tuning workflows in ch-si4684.

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

Software/components/core/src/Bt1035At.cpp

@@ -0,0 +1,68 @@
+/**
+ * @file    Bt1035At.cpp
+ * @brief   Bt1035At implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/Bt1035At.hpp"
+
+namespace core {
+
+namespace {
+
+[[nodiscard]] std::string_view trimAscii(std::string_view text) noexcept
+{
+    while (!text.empty()
+           && (text.front() == ' ' || text.front() == '\r'
+               || text.front() == '\n' || text.front() == '\t')) {
+        text.remove_prefix(1U);
+    }
+    while (!text.empty()
+           && (text.back() == ' ' || text.back() == '\r' || text.back() == '\n'
+               || text.back() == '\t')) {
+        text.remove_suffix(1U);
+    }
+    return text;
+}
+
+} // namespace
+
+std::string buildBt1035AtLine(Bt1035AtCommand command)
+{
+    switch (command) {
+    case Bt1035AtCommand::Ping:
+        return "AT\r\n";
+    case Bt1035AtCommand::AuxLineIn:
+        return "AT+AUXCFG=1\r\n";
+    }
+    return "AT\r\n";
+}
+
+std::array<Bt1035AtCommand, kBt1035BootInitCommandCount> bootInitSequence() noexcept
+{
+    return std::array<Bt1035AtCommand, kBt1035BootInitCommandCount>{
+        Bt1035AtCommand::Ping,
+        Bt1035AtCommand::AuxLineIn,
+    };
+}
+
+Bt1035AtResponseKind parseBt1035AtResponse(std::string_view line) noexcept
+{
+    const std::string_view trimmed = trimAscii(line);
+    if (trimmed == "OK") {
+        return Bt1035AtResponseKind::Ok;
+    }
+    if (trimmed == "ERROR" || trimmed.starts_with("ERROR")) {
+        return Bt1035AtResponseKind::Error;
+    }
+    return Bt1035AtResponseKind::Unexpected;
+}
+
+} // namespace core