refactor(createDataTable): drop items option for registry pattern

[johnleider]

Summary

createDataTable predated the registry-pattern convention used by every other collection composable in v0. The items: MaybeRefOrGetter<T[]> option put row identity outside the composable's registry, forced reactivity plumbing on the caller, and split ownership between the caller's array and the table's internal state. This PR drops items and itemValue and routes row management through the inherited registry surface.

Convention codified

PHILOSOPHY.md §6.10 "Collection composables: no items option" plus a checklist line in .claude/rules/composables.md and .claude/rules/new-feature-checklist.md. The PHILOSOPHY entry lists createDataGrid as also compliant in anticipation of the follow-up grid PR (#174) that adopts the same surface inherited via spread.

API change

// Before
const table = createDataTable({
  items: users,
  columns,
  itemValue: 'id',
})

// After
const table = createDataTable({ columns })
table.onboard(users.map(value => ({ id: value.id, value })))

Internally, createDataTable now owns a createRegistry({ events: true, reactive: true }) and spreads its surface into the returned context, so consumers get register / onboard / unregister / upsert / clear / get / move / etc. for free.

• DataTableContext<T> now extends RegistryContext<DataTableTicketInput<T>, DataTableTicket<T>>.
• allItems / filteredItems / sortedItems / items keep their existing Readonly<Ref<readonly T[]>> shape — they project ticket.value through the pipeline.
• Adapters (Client / Server / Virtual) are unchanged; they read context.items as before.
• size is re-declared as a getter after the registry spread (otherwise { ...registry } freezes it at construction).

Migration shape

Reactive items sources are no longer auto-synced. Callers either drive register / unregister themselves, or watch a source and do clear() + onboard() — the canonical server pattern is documented in the ServerDataTableAdapter section of the docs page.

Co-author credit

Built by parallel agents per the project's team convention:

• dev-table — source + tests
• docs-table — docs page + 4 examples
• philosophy — PHILOSOPHY §6.10 + rules

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commit: a9173fc