Replace Streams fusing XML documentation placeholders

[Aaronontheweb]

Streams fusing and internal DSL APIs had 1,341 XML documentation placeholders in 568 blocks across nine files. Document graph assembly and interpreter lifecycle, boundary owners and port state, queued-event budgets, graph logic creation, internal operators, substream grouping/splitting, and source/sink protocol callbacks. Preserve useful prose and leave executable source, signatures, attributes, nullable directives, ordinary comments, tests, configuration, dependencies, and API approval files unchanged.

Luna authored the docs. Independent Sol review covered all nine files and rechecked every correction, including materialized-value and shape contracts, demand versus batching thresholds, iterator replacement, queue/task outcomes, actor message return values, group key accounting, segment cancellation, and lazy-flow selection versus materialization. A separate 31-class author audit checked every changed Ops constructor/port/CreateLogic block against its owning implementation. This is an independent PR against dev.

Validation:

• Exact assignment scan: zero XML documentation placeholders. Roslyn source comparison preserves executable tokens and all non-documentation trivia/directives.
• Final Streams Release build passes with warnings as errors. Compiler XML member IDs are unchanged; scoped DocFX metadata and rendered documentation pass with zero warnings.
• Existing runtime coverage is carried from the source-equivalent Core documentation candidate: all 34 selected project suites have passing results (8,023 passed, 523 existing skips, zero failed tests), including 1,878 Streams unit tests and 33 existing Streams skips. The original Incrementalist command failed on framework propagation to a source generator; Serialization V2 passed 566 tests with the corrected invocation. CI runs required affected-project checks.
• Slopwatch reports existing catch findings; no runtime code, suppressions, or baselines were changed. Actual IDE tooltip rendering was not exercised. Other independent Streams areas retain their original placeholders on this branch.

Combined local verification with all 13 batches also passes: 22 affected Release builds with warnings as errors, 33 API approvals, zero compiler-XML placeholders, and zero rendered-documentation warnings. Full metadata retains the same 576 baseline warnings; Slopwatch has no introduced findings. No PRs were merged by the agent.

The agent will not attempt to merge campaign PRs.