Add beta/nightly firmware OTA channel

[TheAngryRaven]

What

A second OTA channel for flashing debug builds from your phone at the track, living alongside the existing production channel on gh-pages.

New beta workflow (.github/workflows/beta.yml): on every push to the BETA branch, builds both XIAO nRF52840 variants and publishes them to a beta/ subtree on gh-pages. No GitHub Release, no version tag — these are throwaway nightlies.

• Two manifests, two channels: prod stays at the Pages root (manifest.json, firmware/<version>/…); beta lives under beta/ (beta/manifest.json + flat assets). The deploy's keep_files: true means the channels never clobber each other.
• Latest-only retention: a single flat beta/ slot with stable filenames, overwritten each push (no per-build history — that was your call).
• beta/manifest.json mirrors the prod manifest shape (same builds[model] → {dfuZip, appBin, appCrc32, appSize}) so the web client reuses one parser, plus channel/commit/branch markers and beta/ URLs.

Beta builds self-identify over BLE: project.h now stringizes an optional -DFIRMWARE_VERSION_OVERRIDE=<token> build flag, and the beta workflow stamps <base>-beta.<gitsha> (e.g. 2.2.1-beta.abcdef0). So you can read the version off a device's DIS and know the exact commit on it. Normal builds (prod release, plain IDE, compile-sketch CI) leave the flag undefined → unchanged. The OTA image descriptor's version field was widened 16 → 32 bytes to hold the longer strings.

How to turn it on

1. Merge this PR to master.
2. Create the BETA branch off master (it'll carry beta.yml). Every push/merge to BETA then publishes a nightly.
3. You can also trigger it manually from the Actions tab (workflow_dispatch) once beta.yml exists on a branch.

The web side (DovesDataViewer) needs a small addition to read …/beta/manifest.json for its beta channel — that's a separate repo, out of scope here.

Verification

• Confirmed the -DFIRMWARE_VERSION_OVERRIDE stringizer produces "2.2.1" (prod) and "2.2.1-beta.abcdef0" (beta, 18 chars) with gcc.
• beta.yml parses as valid YAML; host unit tests pass (this only touches a build-time #define + a struct field size, no pure-logic change).

Heads-up: build-step duplication

beta.yml's build steps intentionally mirror release.yml's build job. I kept it standalone to avoid touching the working prod pipeline, but that means BSP/library changes must be mirrored in both (noted in a comment at the top of beta.yml). Happy to follow up with a reusable workflow_call build to de-dupe if you'd prefer — left out here to keep this low-risk.

Note on stacking

Branched off master (2.2.1). It touches the FIRMWARE_VERSION block in project.h, which open PR #47 also edits (the 2.2.2 bump) — so whichever merges second will have a trivial one-line conflict on that #define (keep the override guard + the higher base version). Flagging so it's not a surprise.

https://claude.ai/code/session_01JZbL7ygHQcztBpzz4BiVuU

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Generated by Claude Code

[github-actions]

Coverage — host-testable units

📂 Overall coverage

Metric	Coverage
Lines	🟢 217/218 (99.5%)
Functions	🟢 20/20 (100.0%)
Branches	🟡 207/240 (86.2%)

📄 File coverage

File	Lines	Functions	Branches
BirdsEye/crc32.cpp	🟢 30/30 (100.0%)	🟢 4/4 (100.0%)	🟢 24/24 (100.0%)
BirdsEye/dovex_header.cpp	🟢 96/97 (99.0%)	🟢 6/6 (100.0%)	🔴 57/88 (64.8%)
BirdsEye/filename_validator.cpp	🟢 14/14 (100.0%)	🟢 1/1 (100.0%)	🟢 30/30 (100.0%)
BirdsEye/gps_time.cpp	🟢 45/45 (100.0%)	🟢 6/6 (100.0%)	🟢 30/32 (93.8%)
BirdsEye/gps_validation.cpp	🟢 24/24 (100.0%)	🟢 2/2 (100.0%)	🟢 66/66 (100.0%)
BirdsEye/haversine.cpp	🟢 8/8 (100.0%)	🟢 1/1 (100.0%)	⚫ 0/0 (0.0%)