Add an active-thread state source to Nova Blocks

[georgeolaru]

Is your feature request related to a problem? Please describe.
Query and card blocks can select posts, but the stack has no durable source of truth for which editorial thread is active and which posts belong to it.

Describe the solution you would like
Add an active-thread state source to Nova Blocks. It should:

• represent active versus inactive state;
• identify the ordered source post IDs for the active thread;
• be consumable by Query and collection rendering;
• expose a stable agent-authoring and readback path;
• avoid storing presentation markup as state.

Describe alternatives you have considered.
Hard-coded post IDs in page markup and ad hoc categories do not provide an explicit state transition or a reliable single source of truth.

Additional context
The state must support independent readback and cache-safe active/inactive verification.

[georgeolaru]

Deferred (decision 2026-09-25, recorded in master-strategy 2026-09-25-lt-stack-open-issue-calls.md): editorial threads use native pieces. A thread is a taxonomy term plus a core Query; its inactive or empty state is the Query's no-results block, or a deduplicated fallback Query. The chronology toggle (#632) gets built only when a starter needs it. This generic stateful block stays open but is not scheduled.