Read the calendar as days, weeks and years

[monorkin]

A calendar's recordings and a period's are not the same answer. A calendar lists what it
holds, and a recurring event is one row in it; a day, a week or a year expands that row
into the occurrences that fall inside its window. Draw a week from a calendar's recordings
and a weekly meeting shows up once, on the day it was created.

So there are five reads here rather than one, plus the toggle that decides what they cover:

Operation	Route	Answers
CalendarPeriods().Day(date)	GET /calendar/days/{date}.json	one day — now works as the date
CalendarPeriods().Days(startsAt)	GET /calendar/days.json	the window of days from a date
CalendarPeriods().Week(date)	GET /calendar/weeks/{date}.json	the week a date falls in
CalendarPeriods().Weeks(startsAt, centeredAt)	GET /calendar/weeks.json	nine weeks
CalendarPeriods().Year(date)	GET /calendar/years/{date}.json	the year's grid
Calendars().Toggle(id)	POST /calendars/{id}/toggle.json	the selection it left behind

Every period read is scoped to the calendars the reader has switched on, which is what the
toggle changes — and why the toggle answers the selection rather than nothing, so a client
learns what its next period read will cover without reading the calendars again.

A year is the odd one out on purpose. It answers the grid it is drawn as — one entry per
day, plus the events that span more than one — because a year of expanded occurrences is
not what opening a year asks for.

Notes for review

No .json in the modelled URIs for the three date-terminated routes. Smithy refuses
{day}.json — a literal segment cannot contain braces. Those operations are modelled
without the extension and pick it up from withJSONExtension per request, like the other
label-terminated routes already do. The tests assert the wire path, extension included.

Six exclusions dropped from spec/excluded-routes.json, all of them "phase-2".
GET /calendar/days/now stays excluded and its reason is rewritten: it is the same action
as the day read, reachable by passing now as the date.

The route snapshot and the API version stay pinned where they were. These routes were
already in the snapshot — they served HTML — and what changed is that they answer JSON now.
Re-pinning would pull in every unrelated route added since the pinned SHA and ask this PR
to write exclusions for work it knows nothing about. AGENTS.md ties the service version
date to the snapshot pin, so that stays put too.

Bumped to 0.16.0 — #111 took 0.15.0 while this was open.

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