fix: use leaf graph path for stdio suggestion

[branchseer]

User-facing changes

• For top-level sequential tasks (vp run build1 && vp run build2) with cache disabled: stdin is interactive, and outputs are naturally grouped because there is no parallel execution.
• For tasks that can be parallelized, like vp run -r build, and tasks with cache enabled: outputs are piped and collected, and interactive stdin is disabled. We will print outputs one by one after each task is finished.

Technical changes

• Replaced the labeled reporter's global spawn_leaf_count stdio heuristic with per-leaf evaluation based on graph structure along each LeafExecutionPath.
• Added LeafExecutionPath::all_containing_graphs_single_node(...) to determine whether a leaf is inside only single-node graphs (root + expanded ancestors).
• Moved stdio suggestion selection to new_leaf_execution(...), storing the computed suggestion on LabeledLeafReporter and using it in start(...).
• Removed now-obsolete count_spawn_leaves(...) logic and rewrote labeled reporter unit tests to validate graph-path criteria and ancestor behavior.
• Added a new e2e fixture stdio-graph-criteria that exercises mixed && + expanded graph cases and verifies tty behavior via snapshots.
• Updated stdin-inheritance fixture documentation to match the new criteria wording.

Validation

• cargo test -p vite_task labeled_reporter
• cargo test -p vite_task_bin --test e2e_snapshots -- stdio-graph-criteria
• cargo test -p vite_task_bin --test e2e_snapshots -- stdin-inheritance

[branchseer]

• fix: use leaf graph path for stdio suggestion #167 👈 (View in Graphite)
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