v0.12.0

Phase 7: guaranteed single termination point + milestone tasks

• A duration-0 task with no resources is now a valid milestone task (the
  same convention the engine's own synthesized FINISH node has always
  used) - E_NO_RESOURCE is exempted, E_BAD_DURATION allows 0.
• A non-critical chain that reaches project completion with zero slack
  (no room for a feeding buffer) now connects directly to the synthesized
  FINISH milestone instead of being silently left disconnected.
• check_schedule gained a new connectivity check (E_DISCONNECTED_TASK)
  as a defense-in-depth safety net.
• Docs: new "Guaranteed Single Termination Point" section in
  network-layout.md, milestone-task guidance in formats.md, and several
  stale doc fixes (validation code table, legacy CSV column names).

See PLAN.md's Phase 7 section for full implementation details.