Generate the crossing animation instead of storing 2 KB of bitmaps

[TheAngryRaven]

Summary

Reclaims 2,048 bytes of flash on an image currently sitting at 99.4% of the OTA cap.

The two "calculating" frames drawn while the timer is inside a crossing zone were hand-stored 1 KB PROGMEM bitmaps. Decoding them shows both are pure block patterns — eight 16×16 px cells, confined to the odd 16 px row bands, with the two frames offset by one cell so alternating them scrolls sideways. Spending 2 KB of flash to express that is a bad trade, so they are now emitted by a new host-tested crossing_pattern unit and drawn with eight fillRect() calls.

The bird splash is untouched — that's real artwork, not a pattern, and stays a bitmap.

Equivalence is proven, not assumed: crossing_pattern_test.cpp pins the original 2 KB as goldens, rasterizes the generated rectangles into a 1024-byte 1bpp buffer, and memcmps all 128×64 pixels of both frames. If the geometry ever drifts, the test fails.

One thing worth knowing, since it surprised me: the original bitmaps only ever lit the odd row bands — the top and third 16 px bands were entirely blank in both frames, so it's a two-row block strip rather than a full checkerboard. I reproduced that exactly rather than "fixing" it. If it was meant to be a full checkerboard, that's now a one-character change (cy = 1; cy < rows; cy += 2 → cy = 0; cy < rows; cy++ with the phase folded in) plus a golden update — say the word.

Type of change

• Bug fix (no user-visible behavior change beyond the fix)
• New feature / behavior
• Refactor (no behavior change)
• Tests only
• CI / tooling / docs
• Breaking change (track files, log format, BLE protocol, or a removed mode)

How it was verified

• Host unit tests pass (ctest --test-dir tests/build) — 297 cases / 4,269 assertions, 0 failed, including the two golden byte-identity proofs
• clang-tidy clean — deferred to CI
• Compiles for the XIAO nRF52840 Sense — deferred to CI (the flash delta will show in the size report)
• Tested on real hardware — the golden test makes the raster provably identical, but a glance at the crossing animation on-device is still worth it
• Simulator: full native suite green (6/6 — boot soak, determinism, goldens, both lap oracles, two-session carryover); it compiles the real display_pages.ino, so the new draw path is exercised there.

Checklist

• CHANGELOG.md updated under [Unreleased] (if user-visible)
• ARCHITECTURE.md / CLAUDE.md updated (if a module or interface changed) — new pure-unit row, images.h description
• New testable logic has a matching test in tests/
• Branch is focused

Related issues

Follow-on from the flash-budget work in #116 / plan 0004 — every KB counts until the staging region goes away.

🤖 Generated with Claude Code

https://claude.ai/code/session_01HnTP6BdA9xjLR5hSWE9frb

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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%)