fix(client): decouple runtime cluster add from readiness

[SunSi12138]

Closes #652

Summary

• Make runtime AddClusterAsync commit on local lifecycle/publication facts instead of remote readiness.
• Split Add and Replace candidate activation policy: Add starts the child runtime and publishes; Replace keeps connect/ready-before-swap behavior.
• Allow Add while the parent lifecycle is Running even when the legacy aggregate connectivity state is Connecting.
• Capture the compatibility ConnectAsync slot batch at operation start so a later Add is not grafted into, extended by, or stopped by that older batch.
• Preserve Created-state Add behavior: prepare/publish locally without proactively connecting until the parent runtime starts.
• Preserve route/connection-budget preflight before child Start and atomic local revalidation/rollback before publication.
• Migrate integration/package consumers that require immediate RPC availability to the canonical StartAsync + scoped WaitForReadyAsync flow instead of relying on Add completion as a readiness signal.
• Keep [api][multi-cluster] 为 Add/Replace/Remove 提供各自的 structured result contract #656 structured Add/Replace/Remove result contracts out of this PR.
• Keep development versions unchanged; no package/project/assembly/file/diagnostics schema version bump.

Lifecycle / publication semantics

Running Add now follows:

local validation + route/budget preflight
-> prepare candidate
-> child StartAsync
-> atomic local revalidation
-> publish cluster/routes
-> transaction committed
-> child connectivity supervisor continues independently

A first dial/handshake failure may happen before or after parent publication and does not make a locally valid Add fail. The published child can remain Connecting / Reconnecting / NotReady until its own supervisor converges.

Caller cancellation or parent lifecycle closure before publication rolls back and stops the started candidate. Cancellation after publication does not revoke coordinator ownership.

Created-state Add remains local-only: it prepares/publishes the slot without proactively starting connectivity; later parent StartAsync starts child runtimes normally.

Compatibility ConnectAsync

A legacy ConnectAsync captures its slot snapshot when the operation starts. Concurrent Add publication:

• does not extend or rewrite that captured batch;
• starts the new child under its own runtime supervisor when the parent lifecycle is Running;
• does not let an old compatibility failure stop a newly published child;
• projects aggregate State/Readiness from the latest snapshot after the old operation completes.

Pure legacy Created + Connecting mutation rejection remains unchanged. The Running + aggregate Connecting exception is scoped to Add only; Replace/Remove are not broadened by this PR.

Add vs Replace

• Add: Start -> publish -> background readiness convergence.
• Replace: retains Start/Connect -> ready -> atomic swap -> retire old availability-first behavior.

The implementation uses separate activation helpers so future changes cannot accidentally couple these policies again.

Regression coverage

New/updated tests cover:

• Running Add publishes a started candidate without waiting for Ready.
• Immediate first dial failure still commits Add and leaves the child NotReady / parent Degraded.
• Running + legacy aggregate Connecting permits Add.
• An in-flight compatibility ConnectAsync batch does not grow when a later Add publishes.
• Failure of the old compatibility batch does not stop a slot published after its snapshot.
• Pre-publication caller cancellation after child Start rolls back and disposes the candidate.
• Parent Stop winning after candidate Start but before publication rolls back the candidate and leaves the snapshot unchanged.
• Post-publication caller cancellation does not undo ownership.
• Concurrent same-key Adds remain serialized through the publication boundary.
• Ready + added NotReady projects aggregate Degraded; an added NotReady cluster as the only published slot projects aggregate NotReady and later converges without another ConnectAsync call.
• Replace retains connect-before-swap semantics and unavailable replacement candidates preserve the old authoritative cluster through existing coverage.
• Integration/TLS/package smoke callers that issue RPC immediately after runtime Add now explicitly wait for scoped readiness.

Validation

Implementation base: exact dev d5a74be2bc20080ccc95ff688be90f19b7625cce.

Development/focused validation:

• exact-base / intended-file scope guards: passed
• git diff --check: passed
• maintainability debt gate: passed without new baseline allowance
• version/schema mutation guard: passed
• Release Unit coverage after final test additions: 1693 tests on the formal PR path
• serialized Integration validation after lifecycle/readiness callsite migration: 451/451 passed

Final exact-head candidate: 5fbfb0959ae96fbae4aea8b497c3125f067e7208.

Formal PR CI on that exact SHA:

• PR Fast #1507: passed — project-reference boundary, formatting, maintainability, Release build, generated-assembly dependency check, Unit, Generator, LoadTest, allocation gate.
• PR Quick #2844: passed — formatting/maintainability, Debug + Release builds, Unit, Generator, LoadTest, 451/451 Integration, NativeAOT transport/topology smoke, pack/package contract verification, NuGet package smoke, demo/load/chaos smoke.
• CodeQL #3885: passed.

Final self-review

• Branch is one commit ahead of exact dev and zero behind.
• Final diff contains 13 intended production/test/docs files and no temporary workflow/script files.
• Running Add publication never inspects candidate readiness/connectivity as a commit gate.
• Add and Replace activation policies are explicitly separate.
• Compatibility ConnectAsync uses a captured slot batch and does not own later Add publications.
• Created-state Add still does not proactively connect.
• Pre-publication rollback and post-publication ownership boundaries are covered, including parent Stop and caller cancellation races.
• Aggregate readiness is computed from the latest published snapshot.
• [api][multi-cluster] 为 Add/Replace/Remove 提供各自的 structured result contract #656 structured mutation result work is not included.
• No development version or diagnostics schema version was changed.