ci: pick the build channel from the source tree, not just the target

[TheAngryRaven]

Summary

Fixes the two red compile-sketch jobs on #113 (BETA → master).

compile-sketch.yml resolved LAPTIMER_REF / FEATURE_FLAGS from github.base_ref and github.ref_name only. On the integration PR, base_ref is master and — on a pull_request event — ref_name is the merge ref, never the head branch. So BETA's source tree was compiled with master's channel config: DovesLapTimer pinned to v4.2.0 (which predates CrossingEngine/SprintTimer) and -DBIRDSEYE_ENABLE_SENSOREGG off.

BirdsEye/BirdsEye.ino:45:10: fatal error: SprintTimer.h: No such file or directory
   45 | #include <SprintTimer.h>

Adding github.head_ref == 'BETA' makes a PR whose source is BETA build with BETA's library ref and flags — the channel now follows the code being compiled, not only where it is headed.

The beta publish channel was never affected: beta.yml runs on push to BETA and has been green throughout (its jobs just show up on #113 because they share the head SHA).

Changes

• .github/workflows/compile-sketch.yml — add github.head_ref == 'BETA' to both channel expressions; expand the comment to spell out which of the three refs covers which event.

Unchanged: feature PRs into BETA (base_ref), pushes/dispatches on BETA (ref_name), and every master/release build.

Note for whoever merges BETA → master

The fallbacks stay pinned on purpose, so promoting the channel is still a deliberate act. At that point:

1. Cut a DovesLapTimer release tag carrying CrossingEngine + SprintTimer (BETA has them; v4.2.0 does not) and bump the 'v4.2.0' fallback here, in sim-build.yml, and in release.yml.
2. Decide whether BIRDSEYE_ENABLE_SENSOREGG ships on master.
3. BirdsEye/sim/CMakeLists.txt will conflict — BETA hardcodes GIT_TAG BETA, master (after Release 3.0.2 — FWDFU pre-update (reboot into UF2 mass-storage DFU) #117) uses a DOVESLAPTIMER_REF cache variable. Keep master's version.

Test plan

• yaml.safe_load parses the workflow; both env expressions render as intended
• CI on this PR (base_ref == BETA) stays green — proves the feature-PR path is untouched
• Re-run Beta - autocross and more #113's compile-sketch after merge — both board jobs should go green

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Generated by Claude Code

[github-actions]

Coverage — host-testable units

📂 Overall coverage

Metric	Coverage
Lines	🟢 837/847 (98.8%)
Functions	🟢 89/89 (100.0%)
Branches	🟢 677/743 (91.1%)

📄 File coverage

File	Lines	Functions	Branches
BirdsEye/camera_fsm.cpp	🟢 222/230 (96.5%)	🟢 20/20 (100.0%)	🟡 126/144 (87.5%)
BirdsEye/crc32.cpp	🟢 30/30 (100.0%)	🟢 4/4 (100.0%)	🟢 24/24 (100.0%)
BirdsEye/crossing_pattern.cpp	🟢 15/15 (100.0%)	🟢 1/1 (100.0%)	🟢 12/12 (100.0%)
BirdsEye/dovex_header.cpp	🟢 106/107 (99.1%)	🟢 7/7 (100.0%)	🔴 62/88 (70.5%)
BirdsEye/filename_validator.cpp	🟢 14/14 (100.0%)	🟢 1/1 (100.0%)	🟢 30/30 (100.0%)
BirdsEye/gps_stats.cpp	🟢 25/25 (100.0%)	🟢 3/3 (100.0%)	🟢 8/8 (100.0%)
BirdsEye/gps_status_page.cpp	🟢 25/25 (100.0%)	🟢 3/3 (100.0%)	🟢 24/24 (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%)
BirdsEye/insta360_protocol.cpp	🟢 140/140 (100.0%)	🟢 16/16 (100.0%)	🟡 86/98 (87.8%)
BirdsEye/lap_format.cpp	🟢 18/18 (100.0%)	🟢 1/1 (100.0%)	🟢 9/9 (100.0%)
BirdsEye/sat_bars.cpp	🟢 33/33 (100.0%)	🟢 2/2 (100.0%)	🟢 51/54 (94.4%)
BirdsEye/sd_access_policy.cpp	🟢 9/9 (100.0%)	🟢 3/3 (100.0%)	🟢 18/18 (100.0%)
BirdsEye/sd_format_page.cpp	🟢 25/25 (100.0%)	🟢 3/3 (100.0%)	🟢 25/26 (96.2%)
BirdsEye/sensoregg_protocol.cpp	🟢 44/45 (97.8%)	🟢 7/7 (100.0%)	🟢 33/34 (97.1%)
BirdsEye/sprint_select.cpp	🟢 25/25 (100.0%)	🟢 4/4 (100.0%)	🟢 46/48 (95.8%)
BirdsEye/tach_filter.cpp	🟢 15/15 (100.0%)	🟢 3/3 (100.0%)	🟡 7/8 (87.5%)
BirdsEye/wake_cause.cpp	🟢 14/14 (100.0%)	🟢 2/2 (100.0%)	🟢 20/20 (100.0%)