join: reshape JoinTactic to an iterator of output containers

[frankmcsherry]

Replaces the two-method JoinTactic (defer + fuel-metered work) with a single

fn prep(&mut self, input0: Vec<B0>, input1: Vec<B1>, fresh: Fresh, meet: B0::Time)
    -> Box<dyn Iterator<Item = CB::Container>>;

The tactic now maps two batch lists to an iterator of output containers and holds neither capabilities nor a fuel budget — the same data-to-data division of labor as ReduceTactic. The iterator is the suspension mechanism: a container the driver is free to stop reading.

What moves where

• Driver owns the two per-direction queues, pairs each iterator with its shipping capability, pulls under a split fuel budget, ships each yielded container, and drops a unit when its iterator goes dry.
• "Work remains" becomes driver-observable (an iterator has yet to yield None) instead of a fuel-sign protocol the tactic must self-report. The footgun documented on the old work method is gone.
• Fairness (the two-queue split by arrival direction) moves to the driver.

Sink change

Join closures now receive &mut CB and call push_into directly, rather than a timely JoinSession. A Session is bound to a live operator output and is not locally constructible, so it can't back a self-contained iterator. The arbitrary-container capability #478 wanted comes from the ContainerBuilder itself, not the session, so handing the closure the bare builder suffices. JoinSession is removed.

The EffortBuilder wrapper is also removed: its record counter existed only to meter fuel from a closure's output volume, and the driver now meters fuel from shipped containers' record_count(), leaving the type nothing to do but rename push_into to give.

Cursor tactic

CursorTactic shares its logic closure across units via Rc<RefCell<_>> — each unit is a 'static iterator and cannot borrow the tactic — preserving the single-mutable-state semantics of one closure threaded through every match. Deferred becomes DeferredIter: Iterator; next() plays the same merge-join loop forward (still #[inline(never)]), suspending at container boundaries rather than under a fuel budget.

Tests

Whole workspace builds warning-free. All differential-dataflow tests pass, including SCC and BFS — iterative, recursive-scope dataflows, so capabilities are released correctly when iterators drain (no deadlock).

Open for review

• DeferredIter::next uses mem::take on extract (needs Container: Default), stealing the builder's recycled allocation; a stash-swap would avoid the churn.
• logic's public sink type changed (JoinSession → &mut CB); Mz exposure unverified.

🤖 Generated with Claude Code

[frankmcsherry]

DeferredIter::next uses mem::take on extract (needs Container: Default), stealing the builder's recycled allocation; a stash-swap would avoid the churn.

This is fixable, but seemingly pre-Polonius requires a bit of a dance to return the &mut borrow. A grim loop { .. } and a done variable to bail out of it.