WhaleFlow: Rust executor skeleton for branch/leaf workflows

[Hmbown]

Problem

The workflow script should not execute effects. It should build IR; Rust should own async execution, cancellation, budgets, branch isolation, and reducers.

Scope

• Execute Sequence, BranchSet, Leaf, Reduce, LoopUntil, Cond, and Expand using mock leaves first.
• Enforce branch parallelism caps, cancellation tokens, global run budgets, and per-leaf budgets.
• Add BranchIsolation variants: ReadOnly, PatchBuffer, GitWorktree, TempCopy.
• Add BranchTournament and ParetoFrontier reducer scaffolding.
• Persist run/branch/leaf/control results to TraceStore.

Acceptance criteria

• run_mock_three_branch_workflow passes.
• loop_until_stops_on_pass and loop_until_honors_max_iters pass.
• cond_uses_logged_predicate_result passes.
• expand_respects_max_children and expand_generated_nodes_validate_before_run pass.
• tournament_selects_passing_minimal_branch and pareto_frontier_keeps_diverse_candidates pass.

Source: /Users/huntermbown/Downloads/CodeWhale_v0_9_0_BranchLeafTeacher_Workflows_UPDATED.md and the Claude workflow-mode observation from June 3, 2026.

[Hmbown]

v0.9 stewardship progress update: I opened #2823 as a narrow maintainer PR for the first safe executor slice.

Scope in #2823:

• crate-local MockWorkflowExecutor over WorkflowSpec
• result records for Sequence, BranchSet, Leaf, Reduce, TeacherReview, LoopUntil, Cond, and Expand
• ExpandSpec::max_children plus validation of generated nodes before execution
• pure BranchTournament and ParetoFrontier reducer scaffolding

Intentionally still out of scope: live workflow_run registration, real subagent spawning, worktree application, provider calls, TraceStore writes, replay, and TUI workflow mode. Those still need the cancellation/atomicity/evidence semantics from this issue before closure.

Local verification before opening: cargo test -p codewhale-whaleflow --locked, cargo clippy -p codewhale-whaleflow --all-targets --locked -- -D warnings, cargo fmt --all -- --check, ./scripts/release/check-versions.sh, and git diff --check.

Thanks @AdityaVG13 for the WhaleFlow draft direction in #2482/#2486; #2823 preserves that intent without merging the unsafe runtime/worktree surface yet.