Replace streams Sequence/Flow/Reaktive backends with a round-based Step engine

[slisson]

What

Replaces the streams module's three interchangeable backends (Sequence, Flow, Reaktive) and deferred builder with a single round-based Step interpreter, and removes the Reaktive third-party dependency — while keeping the entire public API unchanged.

A new streams2 module is added as a standalone, dependency-free reference implementation of the engine (with its own tests and README). The same design is then ported into streams to back its richer legacy API.

Why

streams carried three backends because of one capability plain coroutines/Flow can't give cheaply: automatic bulk-request batching for lazy loading of large content-addressed models. Reaktive was there only for the push semantics that let the runtime collect a set of independent requests before forcing any of them.

That requirement is an applicative property (independent branches → same batch round; flatMap dependency → next round). It can be satisfied structurally by one interpreter, eliminating the three backends and the dependency. Prior art: Haxl, ZIO Query, Stitch.

How

• engine/StepEngine.kt — Step IR (Done/Blocked/Failed), applicative/monadic combinators, blocking + suspending round drivers, fetch leaves, and async leaves (for fromFlow/singleFromCoroutine).
• StreamImpl.kt — one backing impl for every IStream cardinality, plus CompletableImpl and the single StreamBuilderImpl.
• SimpleStreamExecutor / BlockingStreamExecutor / BulkRequestStreamExecutor are now thin wrappers over the driver. enqueue(key) is a fetch leaf; batching is structural (per source, per round) rather than tied to an executor-context queue.
• Deleted the four backend builders, the concrete stream classes, the Reaktive helpers, and the Reaktive dependency.

API compatibility / migration

The public surface (IStream.*, IStreamBuilder, IStreamExecutor, IExecutableStream, IStreamInternal, IBulkExecutor, BulkRequestStreamExecutor, all operators/extensions) is unchanged. No downstream module needed API-driven changes. The only other edits remove dead code — unused Reaktive imports in three modelql-untyped files and two unused private helpers in NodeAsAsyncNode.kt — which compiled solely because streams used to re-export Reaktive via api.

Verification

• streams compiles JVM + JS; unit tests pass.
• datastructures and model-datastructure compile untouched; full JVM suites pass.
• modelql-core / modelql-untyped tests pass.
• model-client, model-server, model-server-api, modelql-typed/-html/-client/-server, bulk-model-sync-lib compile (JVM + JS where applicable).
• model-server LazyLoadingTest passes — the access-pattern / batching-correctness test, confirming structural batching reproduces the old Reaktive collect-and-batch behavior.

Reviewer notes — behavioral tradeoffs (intentional, "no incremental emission")

1. iterate / iterateSuspending now fully materialize before visiting (Reaktive previously drained between batches). Raises peak memory for very large server-side iterations; clean follow-up is per-round streaming in just the iterate* drivers.
2. cached() is currently a no-op — fetch-level dedup (the expensive part) is handled by the per-run cache; only pure-recompute memoization is lost.
3. take / skip operate on materialized results (don't prune upstream fetches).
4. SimpleStreamExecutor now also batches per source/round → strictly fewer round-trips, never more.

See streams-redesign.md (repo root) and streams2/README.md for the full design writeup and known limitations.

🤖 Generated with Claude Code

[github-actions]

Test Results

  265 files    265 suites   43m 29s ⏱️
1 528 tests 1 517 ✅ 11 💤 0 ❌
1 538 runs  1 527 ✅ 11 💤 0 ❌

Results for commit fbe71a6.

♻️ This comment has been updated with latest results.

[github-actions]

JVM coverage report

Overall Project	57.98% -0.62%	❌
Files changed	74.2%	❌

File	Coverage
SimpleStreamExecutor.kt	100%	🍏
ModelTreeBuilder.kt	97.86%	🍏
RandomModelChangeGenerator.kt	97.02%	🍏
DeleteNodeOp.kt	96.75%	🍏
SetReferenceOp.kt	95.61%	🍏
AbstractOperation.kt	95.08% -0.82%	🍏
SetPropertyOp.kt	94.95%	🍏
StepEngine.kt	94.66% -5.34%	🍏
MoveNodeOp.kt	90.73%	🍏
SetConceptOp.kt	90.32%	🍏
VersionMerger.kt	89.84%	🍏
RepositoriesManager.kt	89.48% -1.14%	❌
AddNewChildOp.kt	89.43%	🍏
BTree.kt	88.82% -3.29%	🍏
AllChildrenTraversalStep.kt	88.31%	🍏
HistoryQueries.kt	88.17% -2.86%	🍏
BulkRequestStreamExecutor.kt	88% -11.11%	🍏
BindingWorker.kt	85.92%	🍏
DescendantsTraversalStep.kt	82.07%	🍏
CLVersion.kt	78.53% -0.47%	❌
AllReferencesTraversalStep.kt	78.48%	🍏
RolesMigration.kt	78.47%	🍏
HistoryIndexNode.kt	78.41%	🍏
SingleThreadMutableModelTree.kt	72.53%	🍏
CLTree.kt	67.98%	🍏
IStream.kt	63.73% -9.84%	🍏
IModelClientV2.kt	62.2%	🍏
OperationsCompressor.kt	59.56% -6.01%	❌
StreamImpl.kt	57.9% -42.1%	❌
IStreamExecutor.kt	31.49% -11.95%	❌
NodeAsAsyncNode.kt	11.76%	🍏
AddNewChildSubtreeOp.kt	6.82% -32.05%	❌
BlockingStreamExecutor.kt	0%	❌
BulkUpdateOp.kt	0% -10%	❌
LionwebApiImpl.kt	0%	🍏