[PurpleSentinel]

Closes #221. Found by road test: the GPS trigger armed and started the session on pit exit, laps were driven, and no lap time appeared at any point.

The times were never missing

apply_lap_state() fills current, previous and best into the UiSnapshot every service tick, and ui::present() formats all three. Nothing on the device drew that model — ui::present was called only from three host tests. Race borrowed TrackdayScreen, which has no lap labels at all, and says so itself:

Track Day deliberately shows no lap times while running... this screen simply has nowhere to put a lap time.

Same pattern as #18, #204, #214 and #222: built, host-tested, never wired.

The screen

Test Circuit                              RACE

                CURRENT LAP
                 12:34.567

FASTEST                          SESSION LEFT
1:21.002                                 19:45

Separate from TrackdayScreen, not lap fields added to it under a flag. Track Day withholding live lap times is the compliance rule #132 established, not a display preference, and a screen with no lap labels cannot leak one through a mode check that is wrong. running_screen() states the choice once, so the session lifecycle has a single place deciding where a running session goes.

It takes the presenter's DeviceViewModel verbatim rather than formatting anything itself, so --:--.--- before the first lap is the presenter's tested behaviour rather than a fourth copy of lap-time formatting.

Details worth the review

Nine cells, not eight. A lap past ten minutes formats to 12:34.567 — nine characters. An eight-cell row would drop the leading digit. test_ui_foundation now asserts that length.

Repaint discipline, because of #213: cells are rewritten only when their character changes, and the screen is only updated while it is the visible one. The lap in progress changes on every fix; the other two almost never.

kMaximumSizes 6 → 8. Two new numeral sizes could otherwise exhaust the font cache, which fails soft — countdown_font returns nullptr and the glyphs silently do not appear. A silent cap is how the carousel came to be unselectable once before.

Layout is derived, not placed. The supporting row is anchored to the bottom margin; the lap in progress is centred in the band left between it and the title. static_asserts hold that the rows cannot collide, so moving one does not unbalance the other.

Verification

• make check → CHECK_RC=0
• firmware builds clean
• Flashed and reviewed on hardware over three layout iterations
• New assertions in test_ui_foundation.cpp: both lap figures read --:--.--- before anything is measured, 1:23.456 / 1:21.002 when they are, 11:05 for the session clock, and a 12-minute lap formats to nine characters

A lap time still cannot be confirmed end to end on the bench — that needs a drive, and #222 means an interrupted session leaves no record, so the re-test has to be stopped deliberately on the device before power-off.

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