Firmware version from version.txt (0.9.0) instead of git describe

The version is now set by hand in version.txt, so it's readable and only
changes on purpose. git describe gave commit hashes, and always "-dirty"
because the build applies patches to the cspot submodule. Editing the file
is enough; the next build picks it up.

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

CLAUDE.md

@@ -28,6 +28,7 @@ Repo: https://gitea.apointless.space/bsncubed/aes67-ESP32-P4
   - Rev < 3 also limits Espressif's prebuilt audio libraries: `esp_audio_codec` must stay < 2.6 and `esp_audio_effects` < 1.4 (newer versions use P4 assembly that needs rev >= 3; the build fails with a message saying so). Check this for any new Espressif binary component.
 - Embed `web/index.html` via `EMBED_TXTFILES` in `aes67_web`.
 - cspot (Spotify, `external/cspot` git submodule, philippe44 fork, pinned): after cloning run `git submodule update --init external/cspot && git -C external/cspot submodule update --init cspot/bell`. Its nanopb code generator needs, in the IDF Python env: `python -m pip install protobuf grpcio-tools 'setuptools<81'` (after `. export.sh`). Our fixes to cspot/bell live in `components/spotify/patches/{cspot,bell}/*.patch` and are applied automatically at configure time; don't edit the submodule directly, add a patch.
+- Firmware version: first line of `version.txt` (e.g. `0.9.0`). Bump it for a release; editing it is enough, the next build picks it up.
 - Flash over the network (normal way since step 2b; keep USB for recovery):
   `curl -f --data-binary @build/aes67_p4.bin -H 'Content-Type: application/octet-stream' http://p4-aes67/api/ota`
   The board reboots into the new image on trial; check `GET /api/ota` shows the new version with `pending_verify: false`.
@@ -43,7 +44,7 @@ Repo: https://gitea.apointless.space/bsncubed/aes67-ESP32-P4
 - On-die temperature sensor only (driver/temperature_sensor.h).
 
 ## Build order / status
-- [x] 0. Check chip revision and flash size; create IDF project and empty component stubs (layout in aes67-core-base.md). Set up the OTA partition table (two app slots, no factory) and PROJECT_VER from git now, so the layout never changes later.
+- [x] 0. Check chip revision and flash size; create IDF project and empty component stubs (layout in aes67-core-base.md). Set up the OTA partition table (two app slots, no factory) and PROJECT_VER now, so the layout never changes later. (PROJECT_VER comes from `version.txt`, set by hand.)
 - [x] 1. Ethernet: IP101 up, DHCP, IP logged. Ping works.
 - [x] 2. Web server + config store (cJSON in NVS) + embedded index.html; /api/config, /api/status (stub values), /api/reboot.
 - [x] 2a. Finding the device: mDNS (hostname.local + _http._tcp), then LLDP (switch shows name + IP).