feat(wirings): cross-stream tier DataStream wiring + demo — completes the 4-tier matrix (stacks on #8)

[estebanzimanyi]

Fourth and final tier-specific wiring follow-up on PR #5's generated MEOS facade. Stacks on PR #8 (windowed). Adds the cross-stream tier.

With this PR, the 4-tier wiring matrix is complete — every streamable tier in the v4 baseline has a generic wiring class.

What's in this PR

File	Purpose
MeosCrossStreamJoin<L, R, OUT>	Generic ProcessJoinFunction (the operator backing KeyedStream.intervalJoin(other)); time-bounded match window; same-key pairing
demo/MeosCrossStreamDemoJob	Runnable interval-join demo matching two streams on shared regionId key within ±1 minute; emits per-pair overlap events
README.md	cross-stream row marked ✅ shipped

Design — pre-keyed interval-join, watermark-driven matches

Cross-stream is "pairwise across two pre-keyed streams within a bounded time window". Both streams must be keyBy(K)-ed by the same K; only events sharing a key are considered for pairing. The .between(lowerBound, upperBound) declaration bounds the time window; matches fire event-time-aware (watermark-driven).

The adopter-facing signature keeps the slim ContextLike pattern used in MeosWindowedAggregate:

.intervalJoin(other)
.between(Time.minutes(-1), Time.minutes(1))
.process(new MeosCrossStreamJoin<L, R, OUT>(
    (left, right, ctx) -> {
        Pointer leftT  = left.toTGeoPointer();
        Pointer rightT = right.toTGeoPointer();
        if (MeosOpsTGeo.edwithin_tgeo_tgeo(leftT, rightT, 100.0) != 0) {
            return new MeetingEvent(left.id(), right.id(), ctx.getLeftTimestamp());
        }
        return null;  // no output for non-matches
    }));

Final tier coverage after this PR

Tier	Method count (v4 baseline)	Wirable through
stateless	804	MeosStatelessMap / MeosStatelessFilter (PR #6)
bounded-state	797	MeosBoundedStateMap (PR #7)
windowed	161	MeosWindowedAggregate (PR #8)
cross-stream	140	MeosCrossStreamJoin (this PR)
io-meta	195	covered transitively by MeosStatelessMap
sequence-only	14	n/a — inherently non-streamable
Total wirable	2,097 / 2,097 = 100%	5 generic classes

No per-method registration; adopters provide a serializable lambda per use site.

Demo highlights

MeosCrossStreamDemoJob is a 6-event × 2-region cross-stream pipeline:

• Two parallel streams (A, B) keyed by regionId
• .intervalJoin(other).between(-1m, +1m) pairs A-events with B-events in the same region within ±1 minute
• Per matched pair: test overlap via MeosOpsFreeCore.overlaps_tbox_tbox, emit (regionId, vehAId, vehBId, leftTs, rightTs) on match, skip on non-match
• Proves: interval-join semantics, per-key isolation, pairwise MEOS call from a matched pair

Stacking

This PR stacks on feat/flink-windowed-tier-wirings (PR #8). Additive-only: 3 file changes (1 new wiring class + 1 new demo + 1 README row marked ✅).

Compile verification

Locally green: 145 .class files total (140 from PR #8 base + 5 new — 1 wiring class + 2 nested lambda interfaces + 1 anonymous ContextLike + 1 demo class).

What the 4-PR chain delivers cumulatively

Layer	Files	Wirings package methods	Tier coverage
PR #5 codegen	57	2,097 (generated facades)	n/a
PR #6 stateless wirings	+4	+2 wirings	804 + 195 io-meta
PR #7 bounded-state wiring	+3	+1 wiring	+797
PR #8 windowed wiring	+3	+1 wiring	+161
PR #9 cross-stream wiring (this)	+3	+1 wiring	+140
End state	70 total	5 wirings covering 100% of streamable surface	2,097 / 2,097

[estebanzimanyi]

Superseded by the Path-B consolidation: the former 18-deep stack is collapsed into two reviewable topical PRs on top of the merged scaffold — MEOS integration #30 → benchmark #31 — each one clean squashed commit with the generated-facade bulk, dead family-flag profiles, committed target/ artifacts, and invented synthetic corpus removed. Closing as folded into #30/#31.