docs(rfc): explain best-first speculation generation

[behinddwalls]

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 Open and Next flow, preferred and unpreferred assumptions, flip ordering, heap-node expansion, logarithmic scoring, edge cases, correctness properties, and rejected alternatives.

Stack

1. @ docs(rfc): explain best-first speculation generation #513
2. feat(speculation): generator contract and bestfirst impl #446
3. feat(speculation): allocator contract and sticky impl #450
4. feat(speculation): standard composed speculator #451
5. feat(storage): speculation path set store #501
6. feat(storage): path-build link store for per-path builds #502
7. feat(orchestrator): dispatch builds per speculation path #503
8. feat(orchestrator): poll builds and stop the ones nothing wants #504
9. fix(orchestrator): mint distinct message IDs for cancel re-publishes #505
10. feat(orchestrator): re-plan the queue from the Speculator each run #506
11. feat(orchestrator)!: finalize batches from their speculation paths #507
12. feat(orchestrator): compose per-queue speculators and turn speculation on #508