docs(rfc): explain best-first speculation generation - #513
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## Summary ### Why? The best-first generator needs a durable explanation of how it ranks paths lazily without enumerating every dependency-outcome combination. ### What? Document the Generate and Next flow: the strict snapshot contract, preferred assumptions and flip costs, per-head streams that sort flips only when a head is first pulled, the extend/swap subset walk, the global k-way merge over each head’s current best, logarithmic scoring, tie-breaks, per-pull complexity, and rejected alternatives.
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Summary
Why?
The best-first generator needs a durable explanation of how it ranks paths lazily without enumerating every dependency-outcome combination.
What?
Document the Generate and Next flow: the strict snapshot contract, preferred assumptions and flip costs, per-head streams that sort flips only when a head is first pulled, the extend/swap subset walk, the global k-way merge over each head’s current best, logarithmic scoring, tie-breaks, per-pull complexity, and rejected alternatives.