feat(outbox): collect exit verb on BatchOp

[bodymindarts]

Summary

Adds collect to BatchOp as a sibling exit verb to defer() / commit(), completing the verb set introduced in #95.

Handlers entering via consume_in_batch could only exit with commit or defer — an event needing both direct op work now and a contribution to the Batch accumulator had to give up one or the other. BatchOp::collect_with (plus Vec/HashMap collect sugar, mirroring EventCtx) closes that gap: a defer-like exit that also contributes an item, applied by the handler's flush inside the same batch transaction when the batch lands.

let mut op = ctx.consume_in_batch().await?;
insert_row_in_op(&mut op, n).await?;   // direct op work, per event
Ok(op.collect(summary_update(n)))      // coalesced contribution, applied at flush

Design notes

• The runner is untouched. Outcome::Collect already lands like Defer, and flush_batch already handles op+items batches — the whole change lives in ctx.rs.
• No double counting. The exit only bumps tracker.collected; consume_in_batch already counted the event toward max_batch_size. The new test pins this: with max_batch_size == N, a double count would split the burst into two flushes.
• Not on IsolatedOp — deliberately. A collected item lands with a future batch flush, not with the isolated op's commit, so a collect exit there would silently violate the isolation contract.
• Compat: BatchOp<'inv> → BatchOp<'inv, B = ()>. The defaulted param keeps existing BatchOp<'_> mentions compiling; only code naming BatchOp explicitly inside a non-unit-Batch handler (a combination made possible yesterday in feat(outbox)!: collect/flush statement batching, two-stream runner, sealed op guards #95) could notice. Effectively additive — no !.
• Replay contract unchanged: on a failed batch the op work rolls back and replayed events re-collect their items.

Also includes a standalone commit fixing two pre-existing clippy lints in tests/post_persist_hook.rs (type_complexity, await_holding_lock) that were blocking nix flake check.

Testing

• New integration test batch_op_collect_contributes_item_and_defers: each event writes a row directly into the shared batch op and collects an offset item; asserts both land (one batch, one flush, one checkpoint) and that events count once toward max_batch_size.
• Full nix flake check (fmt, clippy -D warnings, deny, tests) passes; all 31 tests in the two touched suites green.

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