feat(ui): show the overrun counting up and route the rest period

[PurpleSentinel]

Part of #142 — the overrun and rest scope. The review screen and the deliberate-stop
confirmation remain open there, so this deliberately does not close it.

The gap

A session that ran out simply stopped. The countdown sat at 00:00 with nothing to say it had
ended — on a track day, the moment a driver most needs telling — and the rest period they had
configured never appeared at all.

Nothing was missing from the state machine. SessionController already modelled
overtime, session_overrun_ms, rest and rest_remaining_ms, host-tested since the rest
workflow landed. What was missing was routing: the router reduced six states to one boolean
and sent everything that was not running or overtime to the dashboard.

What it does now

Phase	Clock	Line beneath	Colour
Session	counts down	lap estimate	green → red ramp
Overrun	counts up	OVER RUN	deep purple
Rest	counts down	REST	green → red ramp

Rest is drawn on the same screen with the same fixed cells and the same ramp, because it is
the same kind of thing: a clock the driver is watching run out.

Colour chosen on contrast, not on name

Candidate	Contrast vs the black panel
#6A0DAD literal "deep purple"	2.27:1
#8A2BE2 blueviolet	3.52:1
#9A4DFF chosen	4.88:1
ramp red #FF3B30	5.92:1
ramp green #2FD16D	10.46:1

Every other state on this screen sits between 5.9 and 10.5. A 10 mm numeral at 2.27:1 is hard
work in daylight, so the literal answer was the wrong one; #9A4DFF still reads as a deep
saturated purple. Trivial to darken if it looks bright on the panel.

Two decisions worth reviewing

No sign on the overrun clock. The field is five fixed cells, and a sixth would need 664 px
of a 600 px panel. The colour and the caption already say which way it counts.

Double tap passes through review. confirm_stop lands there by design, so the tap
carries on to rest programmatically rather than rearranging a tested state machine — a review
screen later inserts itself at that point. If review ever becomes a screen the driver sees,
this flow gains a stop before rest.

Double tap

Two taps rather than one because this ends a session and a stray touch in a moving car must
not. The first tap is still delivered as a press: holding it back pending a possible second
would delay every press by the 400 ms window, and the screens that act on a double tap ignore
single presses anyway.

Verification

35 host suites and 70 simulator tests, with new coverage for the counting-up format, the
caption per phase, the ratio and ramp during rest, and that the overrun colour matches no
point on the ramp.

End to end on the panel:

session start: 0 over 60 s
state=running   remaining=59935  overrun=0     rest=-1
state=overtime  remaining=0      overrun=67    rest=-1     <- by itself, at zero
session phase: overtime -> rest                            <- double tap
state=rest      remaining=0      overrun=6384  rest=1199999
session phase: rest -> ready                               <- double tap
state=ready

The 60 second session ran 60,010 ms, and there were no accidental transitions in the
seconds either side of each tap. Probe removed and its absence confirmed on the final image;
no watchdog triggers.

Still open on #142

The review screen itself, and the deliberate-stop confirmation UI. #142 stays open for them.