Stack

Stacked on activity-only #1691 (which depends on extraction #1686). The existing branch name dev/sticky-activities-v2 is retained to preserve this PR.

Published commit: 20d0d89, exactly one commit above review/sticky-activity-polling at 3b8e0ca. This PR targets that branch and shows only the incremental stable-destination/isolation layer.

Incremental scope

• Optional stable sticky destination shared by workflow and session activity polling.
• Sticky-only construction in lifetime single-session mode, with no independent normal pollers or ratio splitting.
• WorkerSessionOptions on both poll request types.
• Eager workflow starts disabled in single-session mode so they cannot bypass binding.
• Replay clears runtime-only placement options.
• Fixed activity capacity one is allowed in sticky-only mode; paired mode still requires two.

This layer does not restore resource-based activity support or the removed polling-plan/source abstractions. The design document remains local-only. No broader session API redesign is included.

Validation

• Core lib suite: 506 passed, 2 ignored.
• Tests cover single-slot mode, larger fixed polling budgets, autoscaling budgets, both task types using the same stable name, no normal polls, eager workflow rejection, and replay with no explicit sticky name.
• cargo lint passed; protobuf all-features unit/doc tests passed; formatting and diff checks passed.
• Resource tuner and local-only document verified unchanged relative to Add workflow-affine activity polling without isolation controls #1691.
• No live-server isolation test: Add gated routing foundation for workflow-session activities temporal#12630 explicitly rejects single-session isolation. Keep this draft deferred until supporting server behavior exists.

Preservation

backup/session-affinity-before-restack preserves the old published f0971e2 revision used by Python #1979. Python is unchanged by this restack and still needs a later pin update.

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