fix(dx): silence becomes visible — doctor measures time since the last reading

[OrangePeachPink]

Refs #1710 — item 3's minimum bar only. Does not close the issue; items 1 and 2 are maintainer decisions and the app-launch surface is Data's.

What a check can actually fix

Six days of silence looked identical to six days of health.

That's the fixable part. gaps_by_device couldn't have caught it, and it's worth saying why rather than leaving it looking like an oversight: it's window-scoped, so a six-day hole isn't merely unflagged — it isn't even in a "last 24h" view to be flagged. The data knew; nothing was asking the question.

Grading, on the records-lane precedent

#1718 established the right axis for this class, and the underlying fact is identical — readings are irreplaceable, so age, not count:

state	grade	why
no readings at all	ok	a contributor with no hardware is not broken
hours	warn	not collecting right now is an ordinary state
days	FAIL	every hour in that window is gone for good

A warn for the six-day case would have been a warn in a list of warns — which is exactly how the silence stayed invisible.

Measured at the main worktree, never this checkout

logs/ is gitignored, so a worktree has none. Measuring locally would report a six-day silence that reflects only where the agent happens to be standing — an alarm manufactured by the tool, which is worse than the silence it was built to catch. Same lesson as #1688, one layer down; it reuses that check's main_worktree().

Live, from this worktree:

[ok  ]  collection   last reading 0m ago (measured at C:\Users\PV\dev\plants)

The 6.3-day case is a test with an injected clock — I can't make a real collector go silent for six days on demand, and I'd rather say that than imply I measured it.

What this does NOT do — so a green doctor isn't read as coverage

• Not "on next launch the app should SAY". That surface is serve.py, which the justfile marks as Data's lane. Doctor covers the operator who asks; the launch banner covers the operator who shouldn't have to. Routing below.
• Nothing about start-on-boot (item 1) or return-to-power (item 2). Both are machine-configuration decisions the issue explicitly reserves for you, and neither is code in this repo.

just check green; 41 doctor tests.

— DX