syncbat: add atomic typed batch append effect capability

[heyoub]

Problem

syncbat::EffectBackend and EventAppendHandle currently mediate only one durable append at a time. The store backend can lower a typed event to Store::append_typed, while BatPak itself already exposes atomic Store::append_batch[_with_options].

A downstream operation with a large, fully planned event set therefore has two bad choices:

1. append each item separately, losing atomic batch durability and paying one writer round-trip per item; or
2. call the Store batch API directly, bypassing the runtime-owned effect capability and falsifying the invocation's observed effect row/receipt semantics.

The second choice is not acceptable: the effect backend is documented as the only path through which a handler may reach durable effects.

Requested neutral contract

Add an object-safe atomic typed batch capability to syncbat:

• an EventAppendHandle batch method and matching EffectBackend method;
• per-item coordinate, kind/version, canonical payload, append options/idempotency, and causation metadata;
• preflight validation against every declared append target before submitting anything;
• StoreEffectBackend lowering to BatPak's atomic batch API;
• per-item AppendReceipt values returned in input order;
• observed effect state updated only after the batch commits;
• all-or-nothing behavior for encode, declaration, gate, store, and receipt failures.

The carrier must remain object-safe and must not require domain vocabulary.

Proof requested

• mixed-coordinate and mixed-kind typed batches;
• duplicate idempotency keys returning the original receipts;
• declaration denial before any append;
• injected failure before append and during store submission;
• crash/reopen witness proving either the complete batch or none;
• observed effect row remains empty on failure and records the batch only after success;
• equivalence of single-item batch bytes/receipt with the existing typed append path.

Downstream containment

The downstream keeps its correct sequential effect-mediated append loop rather than bypassing syncbat. A narrow compatibility seam can switch to this API when released.