Flatten WeakTopologicalOrdering into a single entry vector

[tlively]

Replace the recursive std::variant/Cycle representation of
WeakTopologicalOrdering with a single contiguous std::vector
where cycle ends store the header block and jump target index. This
avoids per-cycle vector allocations and recursive evaluation in
WTOWorklist::run.

Benchmark results across 16 WebAssembly modules (3 iterations,
interleaved):

• --constraint-analysis:
  • Geomean: 1.606s -> 1.592s (-0.9%)
  • Total time: 64.05s -> 63.67s (-0.6%; faster on 11/16 modules)
• --rse:
  • Geomean: 0.871s -> 0.852s (-2.2%)
  • Total time: 26.03s -> 25.39s (-2.5%; faster on 12/16 modules)