Replace Cluster Tools XML documentation placeholders

[Aaronontheweb]

Cluster Tools had 382 XML documentation placeholders in 213 blocks across 17 files. Document cluster clients, receptionist registration, distributed publish/subscribe, singleton manager/proxy behavior, settings, and protocol messages. Preserve useful prose and ground each description in implementation, callers, and contracts. Runtime source, signatures, attributes, nullable directives, ordinary comments, tests, configuration, dependencies, and API approval files remain unchanged.

Luna authored the documentation. Independent Sol review covered the complete assigned batch and rechecked all corrections. This PR is independent against dev.

Validation:

• Roslyn source comparison preserves executable tokens and all non-documentation trivia and directives; exact assigned-file scan has zero XML documentation placeholders.
• Final affected Release builds pass with warnings as errors. Compiler XML member IDs are unchanged, and 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). The original Incrementalist invocation failed on a framework propagation build issue; Serialization V2 passed all 566 tests with the corrected command. No test or project changes.
• CI still runs the required affected-project checks. Actual IDE tooltip rendering was not exercised.

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.