Fix parallel-branches branch middleware to run in subgraph state space

[chris-colinsky]

Summary

Branch middleware on a parallel_branches node ran the §11.4 outputs projection inside the middleware chain, so the middleware effectively operated in the parent (post-projection) state space rather than the branch subgraph's state space that pipeline-utilities §11.7 specifies. That was invisible until a branch middleware returned a cross-space partial update: RetryMiddleware re-invokes the chain rather than returning one, but FailureIsolationMiddleware's degraded_update (written in the subgraph's fields) does, and it was misapplied to the parent state and tripped extra-field validation.

The fix moves the projection outside the chain. The middleware now sees the subgraph-space partial, and the parent projection runs on whatever the chain produced: the real subgraph result on success, or a degraded_update on isolation. This matches fan_out, which already operated in subgraph space, and the spec's §11.7 model ("branch middleware wraps the branch's subgraph invocation").

Degrade-path leniency

A degraded_update that does not cover a projected outputs field now contributes nothing for that field (the parent keeps its prior or sibling-branch value) rather than raising, consistent with §11.4's buffer-then-merge model for partial contributions. A hard miss inside the failure-isolation path would defeat the purpose of the primitive.

Scope

The success path, fan_out instance middleware, and node-level placement are unchanged. The analogous fan-out degrade-contribution question (what a degraded instance contributes to a homogeneous collection, plus an extra_outputs keying detail) is a genuine spec question and is being routed to spec separately rather than decided here.

Tests

Four branch-middleware unit tests in tests/unit/test_parallel_branches.py: degrade projects through outputs, success-path guard, degrade-omit skips the contribution, and an (isolation, retry) chain where retry exhausts and isolation degrades.

Validation

• uv run pytest tests/: 1295 passed, 376 skipped
• uv run ruff check ., uv run pyright: clean

[Copilot]

Pull request overview

Fixes parallel_branches branch middleware execution so middleware runs in the branch subgraph’s state space (per pipeline-utilities §11.7) by moving the outputs projection outside the middleware chain, and adds coverage for degrade-path projection/leniency.

Changes:

• Run branch middleware on subgraph-space partials and apply outputs projection after the middleware chain.
• Make degrade-path projection lenient by skipping outputs fields not present in a degraded partial update.
• Add unit tests covering degrade/success projection behavior and (isolation, retry) chain behavior; document fix in changelog.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/unit/test_parallel_branches.py	Adds regression and behavior tests for branch-middleware state-space and degrade-path projection semantics.
src/openarmature/graph/parallel_branches.py	Moves outputs projection outside branch middleware chain and skips missing sub-fields on degrade partials.
CHANGELOG.md	Notes the bugfix and the degrade-path leniency under Unreleased “Fixed”.

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