Workflow Task Chunking: v2

[eamsden]

What was changed

• Add a v2 logical Workflow Task chunker with explicit event classification, pending-Update handling, complete command-batch lookahead, and pagination-safe deferral.
• Select v1 or v2 once per run from the first successful Workflow Task completion. Flagless runs stay on v1; they can adopt v2 through Continue-As-New.
• Keep the writer disabled by default behind TEMPORAL_USE_WFT_CHUNKING_V2; readers honor the durable history selection.
• Fail tasks with unknown first-completion Core flags instead of silently selecting the wrong chunker.

Why?

The legacy heartbeat heuristic can choose different activation boundaries during live execution and replay, especially when WorkflowExecutionUpdateAccepted follows another command or another history page. That can change workflow-visible time and command production, resulting in nondeterminism.

Checklist

1. Closes [Bug] Potential NDE on empty WFT followed by WFT with inbound update #1146. Builds on the investigation in Add WFT Chunking v2 #1281.

2. How was this tested:

   • cargo test -p temporalio-sdk-core --lib
   • TEMPORAL_USE_WFT_CHUNKING_V2=true cargo test -p temporalio-sdk-core --lib
   • cargo lint
   • cargo test-lint

3. Any docs updates needed?

   • Updated the Workflow Task chunking architecture and rollout document.