Step 7.5b1: cspot (philippe44 fork) builds and links on the P4

- external/cspot: git submodule, pinned to philippe44/cspot 3010349
  (bell ed2d6e9); GPL-3.0.
- components/spotify: project component wrapping cspot. bell trimmed to
  what cspot needs (Tremor Vorbis; no codec wrapper, sinks, MQTT, web
  server, fmt, regex; cJSON from IDF). Xtensa biquad assembly filtered
  out (P4 is RISC-V). CMAKE_POLICY_VERSION_MINIMUM 3.5 for CMake 4.
- Patches in components/spotify/patches, applied at configure time:
  nanopb generator works with protobuf >= 5 (MakeClass removed);
  URLParser.cpp missing <cstdio>/<cstring> for GCC 14.
- CONFIG_COMPILER_CXX_EXCEPTIONS=y (cspot needs exceptions).
- spotify_init() only builds a LoginBlob (link check).
- CLAUDE.md: submodule init and IDF-venv Python packages for nanopb.
- Verified on board: "cspot linked: device "P4 AES67", zeroconf info 588
  bytes"; HLS still playing, PTP locked; image 1.83 MB (70% free).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

components/spotify/patches/bell/0001-nanopb-generator-protobuf5.patch

@@ -0,0 +1,18 @@
+diff --git a/external/nanopb/generator/nanopb_generator.py b/external/nanopb/generator/nanopb_generator.py
+index e40f18b..64dad5e 100755
+--- a/external/nanopb/generator/nanopb_generator.py
++++ b/external/nanopb/generator/nanopb_generator.py
+@@ -1463,7 +1463,12 @@ class Message(ProtoElement):
+         optional_only.ClearField(str('extension'))
+         optional_only.ClearField(str('enum_type'))
+         desc = google.protobuf.descriptor.MakeDescriptor(optional_only)
+-        msg = reflection.MakeClass(desc)()
++        # protobuf >= 5 removed reflection.MakeClass (patch for this project)
++        try:
++            msg = reflection.MakeClass(desc)()
++        except AttributeError:
++            from google.protobuf import message_factory
++            msg = message_factory.GetMessageClass(desc)()
+ 
+         for field in optional_only.field:
+             if field.type == FieldD.TYPE_STRING:

components/spotify/patches/bell/0002-urlparser-includes.patch

@@ -0,0 +1,13 @@
+diff --git a/main/io/URLParser.cpp b/main/io/URLParser.cpp
+index 4a50ab1..af679ec 100644
+--- a/main/io/URLParser.cpp
++++ b/main/io/URLParser.cpp
+@@ -1,5 +1,8 @@
+ #include "URLParser.h"
+ 
++#include <cstdio>   // sscanf (GCC 14 / ESP-IDF 5.5: not included transitively)
++#include <cstring>  // strstr
++
+ namespace bell {
+ 
+ #ifdef BELL_DISABLE_REGEX