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>

main/CMakeLists.txt

@@ -2,4 +2,4 @@ idf_component_register(SRCS "main.c" "project_cfg.c" "player.c" "audio_ring.c" "
                        INCLUDE_DIRS "."
                        REQUIRES esp_app_format esp_hw_support heap esp_http_client mbedtls esp_timer
                                 aes67_board aes67_health aes67_net aes67_ota aes67_ptp aes67_sdp_sap
-                                aes67_syslog aes67_tx aes67_web)
+                                aes67_syslog aes67_tx aes67_web spotify)