feat: batch options and shared item seeds (4.1.0)

[dasmig]

Phase 3 step 3 of PLAN.md, for entity-generator: batches move to the shared item seeds and take core::batch_options.

• generate_batch and generate_batch_async with a seed now use core::item_seed(call_seed, i) for entity i, the same derivation as every other library (before: the i-th value of rng{call_seed}.split("batch")). Their output for a given seed changes (minor version, as agreed in the plan: nothing else consumes it). Each entity still replays with generate(request, e.seed()). The one known-answer hash of a seeded batch is updated.
• New overloads generate_batch(request, options[, batch_seed]) and for_each_entity(request, options[, batch_seed], sink) over core::run_batch / core::for_each_batch; request.count keeps working until the next major version.
• Vendored core updated to 1.1.0 (DASMIG_CORE_REQUIRE(1, 1)).
• Tests: entities equal the single calls (1, 2, 4, 7 and hardware threads), ranges via first, request.count, options.count and the async batch give the same entities, streaming equals the batch, and a known-answer hash of 10,000 entities. All 293 cases pass with clang 22.
• CI: a ThreadSanitizer job runs the batch tests with four threads. Per-entity contexts own their random engine, so there is no shared mutable state (the mutable engine in component_context is per entity).
• Docs: a "Batch Generation" section in doc/usage.md, a README bullet, Doxyfile at 4.1.0.

🤖 Generated with Claude Code