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>

sdkconfig.defaults

@@ -28,3 +28,6 @@ CONFIG_LWIP_MAX_SOCKETS=16
 # 32 MB PSRAM in the P4 package (hex mode, 200 MHz; 250 MHz needs rev >= 3). Needed for step 7:
 # HLS segment buffers, cspot, decoders. malloc() puts blocks >16 KB in PSRAM; DMA buffers stay internal.
 CONFIG_SPIRAM=y
+
+# cspot (Spotify Connect, step 7.5) needs C++ exceptions.
+CONFIG_COMPILER_CXX_EXCEPTIONS=y