[Foundation] FT-8: Opt-in DAG-parallel target execution

[ChrisonSimtian]

Summary

Add opt-in DAG-parallel target execution. The dependency graph already encodes what can run concurrently; the engine just doesn't exploit it. Default stays sequential.

Findings (file:line)

• BuildExecutor.cs:42 — execution is a serial ExecutionPlan.ForEach(...).
• Fallout.Core/Planning/TopoSort.cs:93-107 — independent nodes are detected per step but only used to flag ambiguity in strict mode; output is a flat linear order, not parallel waves.
• Partition is CI-machine sharding, not intra-build parallelism — distinct feature.

Scope

• Have TopoSort expose ready-sets / waves (nodes whose dependencies are satisfied).
• Replace the serial loop with a bounded concurrent scheduler honoring DependencyBehavior, ProceedAfterFailure, AssuredAfterFailure.
• Gate behind --parallel[=N], default off.

Acceptance criteria

• Independent targets run concurrently when the flag is set.
• Serial execution remains the default and is behavior-identical to today.
• Failure/skip/assured-cleanup semantics preserved under parallelism.

Notes

• Non-breaking (opt-in flag).
• Depends on: FT-3, FT-4, FT-5, FT-6 (de-statification + thread-safe logging must land first).
• Size: L. Own sub-thread of the epic.