fix(weather): guard the single clock reading on the weather view

[prorochestvo]

Two follow-ups recorded during the outlook review, neither of which had an owner until now.

The clock seam (was: the one fix that shipped without a guard)

Hoisting time.Now out of ObtainMeCurrent's per-city loop was correct and untested. Moving
the call back inside passed the entire suite, so the next refactor could undo it in silence
and two cities either side of a midnight would be handed baselines a calendar day apart
inside one response — each one's 16-day window starting somewhere different.

weather.Service now takes the clock seam chart.NewService and digest.NewService already
use: an explicit trailing now func() time.Time, wired as time.Now in cmd/web. No new
pattern invented.

The guard hands it a clock that steps over a local midnight between calls and asserts two
things — both cities were measured against the same instant, and the clock was read exactly
once. Verified by re-inlining the call per city: both assertions fail.

stubForecasts now records the window lower bound of every call, which is what makes the
baseline observable from outside at all.

The log line

The forecast agent gained a deferred= field in #138. Two places quoted the old
healthy-looking line — the decoder comment explaining why an unstorable batch must be an error,
and the same passage in beacon-collection. Both now quote what the collector actually
prints. Nothing parses the format: a grep for the old form outside the frozen plans returns
nothing.

Gate

go vet, scripts/lint-checks.sh and golangci-lint clean; 41 packages green.

Refs #127, #132