Test-build pipeline Phase B, PR 4: extract Partials generation into PartialsManager

[mkarlesky]

Summary

Fourth and final PR implementing Phase B of the test-build pipeline (lib/ceedling/test_invoker/) architecture review, following PR #1217 (Phase A), PR #1218 (PR 1), PR #1219 (PR 2), and PR #1220 (PR 3). Depends on PR 3's consolidation of the two Partial preprocessing methods.

TestBuildExecutor owned every stage from preprocessing through execution -- including the three Partials stages (preprocessing partial headers/sources, then extracting and generating implementation/interface/types content), 446 lines that only share TestPipelineHelpers with the rest of the class, no other coupling. Extracted into a new PartialsManager class (name and location confirmed with you beforehand), mirroring the TestSourceFileDirectiveResolver precedent from an earlier session.

• Promoted directives_only_fallback? into TestPipelineHelpers first -- it's called both by code that's moving (the Partials preprocessing body) and code that's staying (stage_preprocess_mocks, stage_preprocess_test_files), so it needed a home both classes can reach.
• Moved the three stage methods (unchanged) into lib/ceedling/test_invoker/partials_manager.rb, with a constructor pulling only the 9 collaborators they actually use (TestBuildExecutor no longer composes Partializer at all -- confirmed zero remaining references after the move).
• Wired the new class into lib/ceedling/objects.yml and TestPipelineManager's own constructor; TestPipelineManager#build_stage_sequence's stage 6/7/8 lambda bodies now call @partials_manager instead of @test_build_executor -- no change to stage ordering, conditions, or stop-point behavior.
• Verified the DI wiring itself with a local ceedling new + ceedling test:all smoke run (Partials and preprocessing enabled) before touching Docker -- confirms objects.yml has no typos or missing composers, since a DI resolution error would surface immediately at boot.
• Moved the three stage methods' existing unit spec contexts into a new spec/units/partials_manager_spec.rb, retargeted at the new class's own (smaller) constructor -- every example passed unmodified, since the moved code itself didn't change, only its home.
• Updated the README's pipeline-shape section (four classes -> five) and stage 6-8 description to name the new class.

Test plan

• bundle exec rspec spec/units -- 2589 examples, 0 failures (both on host and inside the Docker image)
• Local DI-wiring smoke test: ceedling new + ceedling test:all with Partials and full test preprocessing enabled -- boots and completes cleanly
• Docker (throwtheswitch/madsciencelab-plugins:v1.1.4) spec/system/delta_builds_spec.rb (wondrous_forest Partials cases) -- 16 examples, 0 failures
• Docker spec/system/gcov_deployment_spec.rb's Partials/coverage cases -- 16 examples, 0 failures

This closes out the Phase B test-build pipeline architecture review.

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