Remove the stack timezone: time values carry their own meaning #473

Problem

Every adapter read ownerEntityId and timezone from _config once, at open(), and cached them. ownerEntityId is immutable, so its cache can't go stale. timezone can be patched (stack.patchContent('_config', { timezone })), and after that adapter.timezone and stack.timezone keep returning the old value until the store is reopened.

Dropping the cached property and reading timezone from _config fixes the staleness, but it exposes a deeper issue. The only thing that depends on a stack timezone is commons event's allDay rule ("calendar dates … interpreted in the stack's configured timezone"). _config is owner-only, so once discovery stops carrying timezone, a non-owner viewing a shared all-day event can't interpret it. Making _config readable would be a permission an owner can revoke, or a new special rule, and either way the stack timezone would still be the wrong place for this fact.

Proposal

A time value carries what it means, and a date's form says which of three things it is:

• A date-time with a Z or offset is a moment: the same instant everywhere. Apps show it in the viewer's timezone; "3 hours ago" needs only the current time.
• YYYY-MM-DD is a calendar day: the same date for every reader.
• A date-time with no offset is a wall-clock reading (9am in Lisbon). It names no instant until paired with a zone, so a Type that stores one declares a zone field beside it.

Changes:

• Remove timezone from ConfigContent, the _config@1 schema and every open() option. _config holds only { ownerEntityId }.
• Remove timezone from StackRecordAdapter and its implementations (MemoryAdapter, the shared SQLite adapter, adapter-local, combineAdapters, APIAdapter), and drop stack.timezone. Remove StackIdentity, which becomes the same shape as ConfigContent; newStoreSeed() takes ConfigContent.
• Remove timezone from the discovery response (DiscoveryResponse, fixtures, wire-format.md § Discovery). This is a wire change.
• Commons event: drop allDay. startsAt's form says whether the event is timed or all-day, and endsAt is the first moment or day after the event, in either form (as iCalendar's DTEND).
• Spec: state the rule in data-model.md § Types, and update spec.md § The _config record, adapters.md, portability.md, wire-format.md.

ownerEntityId stays cached: it's immutable, and core uses it for permission checks.

Changeset: minor for core, wire-types, conformance-fixtures, adapter-api, adapter-local, record-adapter-sqlite and record-adapter-do-sqlite.

Follows from #470.